The mitzvah of covering the hair applies only to a married woman. There are many married women who fulfill the mitzvah of covering their hair but many do not even know the significance or understand how important this mitzvah is. Covering the hair is mentioned in the Torah as well.
In Bamidbar, parshat Naso, it says: Then the kohen shall stand the woman up before the Lord and expose the [hair on the] head of the woman; he shall place into her hands the remembrance meal offering, which is a meal offering of jealousies, while the bitter curse bearing waters are in the kohen's hand.
Commentary: He unravels the plaits of her hair to humiliate her. From here [we derive] that a bared head is considered a disgrace for the daughters of Israel.
also in Parshat Korach we learn that the wife of On ben Peleh was saved from suffering the same fate, the same punishment as Korach, because his wife gave him wine to make him sleep and then she went outside her tent and uncovered her hair so that when Korach was coming to get her husband to join him in his rebellion against Moshe Rabbenu, when Korach saw her hair he turned away and did not come near because he knew a man is forbidden from seeing a married woman's hair unconvered.
Rabbi Shimon Bar Yochai, author of the Zohar, promises that whoever covers their hair will merit and bring to their families health, wealth, nachat from children AND from grandchildren! So it even affects the next generation!!
Once a woman marries, she becomes forbidden to any other men. A woman has the responsibility of being very careful that she does not arouse lustful thoughts in another man.
The Zohar explains that in particular the hair of a married woman has a power to arouse lust in men who see her because inside the hair of a married woman is a certain impurity, kelipa, that projects negative energy to others. A man may suddenly find himself with lustful thoughts for a married woman he meets without even understanding why. If a woman does not cover her hair and she arouses lust in other men, this brings with it a lot of negativity for herself, her husband and her family and could in fact negatively affect her own marriage as well.
When a woman covers her hair, the Zohar explains that she covers over and subdues the kelipa and lust, hence bringing purity and kedusha to her household and to everyone around her. This applies even within the privacy of her own home.
Rabbi Hizkiyah said: a confusion of the mind will come upon a man that allows his wife to let the hair of her head be seen by others. A wife who exposes her hair outside to show off her beauty to others causes destitution to visit upon the house. She causes her sons to be undistinguished in that generation and she causes a spirit of impurity to dwell upon the house. [Hair is almost dead and therefore already close to tamei/impure.] What is the root cause of all these? It is the hairs of her head that are seen uncovered. If these results happen [if her hair is exposed] within the house, even more so if it is in a public place [because there she causes men to have sinful thoughts because the uncovered hair of a married women is likened to nakedness] and certainly more so if she is otherwise impudent [showing other parts of her body immodestly. Therefore, it is written "be like a fruitful vine in the recesses of your house" [and not seen outside].
Hair represents the harshest of severe judgments. A woman's hair has a stronger attraction to external forces, and thus is necessary to cover and not reveal the hair since being revealed empowers those impure forces.
Therefore, a woman's hair ideally should be covered even in the innermost parts of her home. If she adheres to this, it is written: "Your children like olive plants." (Psalms 128:3) What does it mean "like olive plants"? Just as an olive tree does not lose its leaf cover in winter or summer and always has more value than the rest of the trees, so should her sons be elevated in respect to other people. Her husband will also be blessed in everything, with blessings Above and with blessings below, with wealth, children and grandchildren. This is what is meant by [the continuation of this psalm]: "Behold, thus shall the man be blessed who fears G‑d" (Ibid. 128:4) and "G‑d shall bless you out of Zion: and you shall see the good of Jerusalem all the days of your life. And you shall see your children's children, and peace upon Israel." (Ibid. 128:5)
Many people have heard the famous story of Kimchit who merited to have seven sons who became Cohanim gedolim in the Beit Hamikdash. The Rabbonim visited her to determine what she did to have this great merit. She told them that the walls of her house never saw her hair uncovered.
A woman covering her hair is not just a private personal choice or event. It is something that directly affects her husband, her children and even her generations!
This is similar to the idea of mikvah: when a woman goes to mikvah, it brings purity to her marriage and creates a proper vessel for the light to reside between husband and wife. If a woman does not observe this mitzvah of mikvah, the light cannot rest within her marriage properly and the impurity from her relationship with her husband could then create various problems (from marriage problems, to fertility problems, to depression , health problems etc. etc.).
It is similar to the idea of an electrical short circuit which disconnects the light from its source. A woman, for example, who does not remove actual food stuck between her teeth before going into the mikvah waters, even if she bathed for hours and even though her mouth is closed while she is under the water, does not receive the spiritual benefit from that immersion and the immersion is not valid. If there is anything that prevents the water from reaching every part of the body, the immersion is not kosher and it is like the short circuit.
Covering the hair is also a vehicle through which a woman brings purity and holiness to her marriage and her relationship with her husband. If she does not wish to cover her hair, she also does not allow that light and holiness to permeate her home and it is like that short circuit which prevents the light from reaching the bulb.
Every mitzvah that we do brings with itself many physical and spiritual benefits. Covering the hair is one that is specifically mentioned in the Zohar and brings tremendous blessings for health, wealth and nachat.
(Just a side note: the hair of the head, according to Jewish mysticsm, is connected to gevurah and when a married woman cuts her hair , she is bringing more chesed and rachamim to her family. The hair does not have to be too short, but it certainly does not have to be long, especially when it is being covered anyway. The idea of a married woman covering her hair, not only cutting it, also is connected to the idea of covering over din and not allowing negative spiritual influences to be available to others).
The Arizal teaches that the hair of the head is of the quality of judgment and perhaps this is the association of hair with ego. Nothing more than ego limits a person. The hair of the head, located as it is on the top of the human body, can be perceived as the keter (literally "crown") of negative human ego. A woman is required to cover her hair on the head but is not to eliminate it. In actuality, the opposite is true. She is discouraged from making herself hairless, given the need for her embodiment of a tamed force of judgment in this world. Women are rooted in the left side of the Tree of Life and correspond to the attribute of gevura (strictness). Her gevura needs to be contained but not eliminated. Men are rooted in the Right side of the Tree of Life, and find their source in the chasadim; therefore, their gevurot/dinim, i.e. the hair of their heads, needs to be eliminated largely but not completely.
How is it that our hair has come to embody the quality of judgment? Kabbala explains that anything with a dominant quality of judgment, like hair, is inherently vulnerable to exposure and damage from the "Other Side" i.e. Evil. That there is evil in the world and that the world is in such a spiritually collapsed state of existence is a result of the fall of Adam and Eve. (DerechHashem 1:3:8) Needless to say, before their fall Adam and Eve did not cover their hair, as they were both naked (Gen. 2:25), free of sexual lust and ego.
We, as fallen human beings seeking redemption, need to respect the situation that we, as part of archetypal Adam and Eve, have collectively created. We need to work with this reality towards Tikun (correction) haolam (correcting the world spiritually).
The Zohar states quite clearly that when we expose the judgments below, we cause a descent of the judgments above into this world below. These then later wreak havoc on our lives.
In the words of the Zohar: "Disruption of spiritual consciousness will befall a man who allows his wife to expose her hair; this [the proper covering of a woman's hair] is one of the foundational principles of modesty. A woman who let's her hair be seen in order to appear attractive causes poverty to her home, spiritual inferiority to her children and causes a negative spiritual influence to reside in the home… If all this in one's own house certainly in public places and certainly other brazen acts of immodesty…."(Zohar, parashat Naso 125b)
The Lubavitcher Rebbe explained and initiated the custom of covering the hair with a sheitel (a wig) rather than only a kerchief or tichel. He said that with a kerchief or tichel a woman may feel embarrassed and simply remove it if she sees someone in the street that she thinks may look at her strangely and she may be tempted to remove the kerchief….or a tichel may slide back exposing some of her hair….and it is harder to fit the hair under a kerchief, especially if it is not so short. Therefore he emphasized the importance of wearing a sheitel (a wig) outside the home.
(In the house one can wear a tichel or kerchief and it is in fact advisable since cooking and looking after children could dirty or mess up the wig and many wigs are very expensive).
But outside the home it is best to cover with a wig.
It is a mystical matter and covering the hair definitely brings purity to the world and subdues kelipa and impurity.
To a woman who asked for the source of this custom to cover the hair, the Lubavitcher Rebbe answered the following:
"You ask about the source of the Jewish custom of women covering their hair. This is not a custom, rather an explicit law in the Code of Jewish Law; nonetheless Jewish customs and practices are very important. Amongst Jewish laws, this law has specific importance, as mentioned in halacha and Zohar.
More surprising is your question, because anyone wanting their marriage to be truly rich and good should embrace this law.
Is there any comparison between your temporary uncomfortable feelings in covering your hair, and the everlasting reward and blessing from G-d Almighty!? This attitude of rejecting covering the hair is a light headed one."
Igros Kodesh volume 23, p.345-346
Some women try to wear long stylish wigs. Certainly one is allowed to do so, but one should keep in mind when choosing a wig that it should look modest. Of course covering on'es own hair is already an act of modesty, but it is also a good idea to choose a wig that is refined looking, not overly attractive that it may cause men to pay attention to it or it may arouse improper thoughts in a man’s mind.
Women should remember that the idea of covering the hair is modesty, and one should choose a sheitel with that in mind.
This secction would be incomplete if we did not mention modesty in dress and behavior as well.
A woman could dress strictly within the guideines of halacha, Jewish law, and yet her clothing could be too tight or too loud colors and could have the opposite affect….so even when buying clothing, one should put emphasis on modesty and still realize she can look beautiful but in a dignified way.
The Lubavitcher Rebbe spoke about modesty and he emphasized the responsibility one has towards others by being modest.
Those who don’t behave modestly do not only defy the path of Torah. Their behavior contravenes basic decency, basic morality, and simple common sense.
When one behaves immodestly by exposing a part of the body that ought to be covered, one’s intent is basically to provoke the evil inclination - the negative side of the other person, even if that intent is subconscious.
This will not help the other person to become a better person. It will not stimulate his mind and make him smarter. Nor will it improve his character traits by positively affecting the respect he accords to his parents, brothers, or sisters, or even to his own wife.
What is the impact of dressing in violation of the requirements of modesty? If until now, the other person’s negative side was hidden, or not excited and aroused - one provokes that person and inflames that side.
(One then shares in the responsibility for the possible sins that could result from the other person being aroused by seeing immodest dress etc. And it is not right to think that it is the other person’s wickedness; it is the other person who is fulfilling his desire, whether through gazing or in another form. One cannot justify this type of immodest dressing by thinking that it is the responsibility of the other person whether he looks or not. One must realize that one must do everything not to provoke another person—not to provoke his negative side but rather to encourage his positive side)
One who behaves immodestly becomes a source of harm, may G–d save us, and not only to one’s own Divine Soul, but also to another person, and still another - in fact, to everyone one meets. What an utterly wrong and degenerate path - may Hashem protect us.
If one would follow the straight path, one would know that the purpose of life is to be a good person, to influence others to follow the proper path, and always to strengthen the good within them.
Some people argue that it is their personal business how they dress or behave.
At the farbrengen of 12 Tamuz 5730, the Rebbe refuted these positions and demonstrated that dressing immodestly causes harm and damage to others and is an act of cruelty. [This applies regardless of how innocent one’s intention to dress immodestly might be (even if one only intends to be more comfortable) since the outcome and effect of dressing immodestly is that one is engaging in a lifestyle that harms and damages others]
A woman’s beauty should be an inner beauty that shines through: actually a spiritual beauty. But the emphasis should always be on looking beautiful within the boundaries of modesty. That is true beauty.
A woman wants people to notice her modesty, not her body or hair. It is the modesty that is the true beauty.
As it says in Kohelet and we read it each Friday night at the Shabbat table in Eyshit Chayil:
Charm is deceptive and beauty is vain….a woman who fears the L-rd is praiseworthy.