The word "hijab" or "ḥijāb" (Arabic: حجاب hijaab) refers to both the head covering traditionally worn by Muslim women and modest Muslim styles of dress in general.
Allah (SWT) has given equal rights to both men and women; He forbids either sex claiming supremacy over the other. Allah (SWT) states in Holy Qur'an: "O mankind! We created you from a single (pair) of a male and a female, and made you into nations and tribes, that ye may know each other (not that ye may despise (each other). Verily the most honoured of you in the sight of Allah is (he who is) the most righteous of you. And Allah has full knowledge and is well acquainted (with all things)." Holy Qur'an (49:13)
Hijab is one of the righteous deeds and it is a sign of honor and equality with men and stands as a shield of protection against evil man.
The woman in Islam is bestowed with respect and honour and a Muslim gives great importance to his wife. Our Holy Prophet Muhammad's (saw) Hadith says the best of you are those who are best in resting their wives.
The status of wife is so respectable that she is not bound to do any house-hold work unless she does it willingly and with affection; but if her husband forces her to do any work; he will have to answer to Allah (SWT) for his conduct. Also, she is not bound to earn and maintain the house of her husband. Her work shall always conform to the laws of Islam (Shariat). Unlike the Western world, the Muslim wife is a queen in her husband's house. The first thing a husband does for his wife is to provide a servant and a cook according to his ability and when she becomes a mother, she receives greatest respect from her husband. Contrary to this, is considered a sin by Allah (SWT).
One of the most common phrases in Islam is the 'Paradise lies at the feet of the mother'. And Allah (SWT) states in Holy Qur'an: "And your Lord has commanded that you shall not serve (any) but Him, and goodness to your parents. If either or both of them reach old age with you, say not to them (so much as) "Ugh" nor chide them, and speak to them a generous word." Holy Qur'an (17:23)

Why hijab in Islam is necessary?