A manual of Islamic law certified by the foremost Islamic institution among Sunni Muslims, Cairo’s Al-Azhar University, as conforming to “the practice and faith of the orthodox Sunni community” explains the guardianship system:

A husband may permit his wife to leave the house for a lesson in Sacred Law, for invocation of Allah (dhikr), to see her female friends, or to go to any place in the town. A woman may not leave the city without her husband or a member of her unmarriageable kin accompanying her, unless the journey is obligatory, like the hajj. It is unlawful for her to travel otherwise, and unlawful for her husband to allow her to. (‘Umdat al-Salik, m10.3).

This is based on a statement attributed to Muhammad, the prophet of Islam:

It is not permissible for a woman who believes in Allah and the Last Day to allow someone into her husband’s house if he is opposed, or to go out if he is averse.

In other words, a woman is her husband’s slave: he controls her movements, and if she dares to get out of line, another Sharia provision that is rooted in a Qur’an verse offers husbands a ready remedy:

Men are the managers of the affairs of women for that God has preferred in bounty one of them over another, and for that they have expended of their property. Righteous women are therefore obedient, guarding the secret for God’s guarding. And those you fear may be rebellious admonish; banish them to their couches, and beat them. (Qur’an 4:34)