Please tell me or explain what is Khul'?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

Khul' is when a wife demands divorce if she does not like her husband for any reason. She can do so giving him back the bridal money he gave her. This is lawful as proved in the Qur'an and the prophetic narration: Allaah says (which means): {… Then if you fear that they would not be able to keep the limits ordained by Allaah, then there is no sin on either of them if she gives back (the Mahr or a part of it) for her Al-Khul' (divorce).}[2: 229].

The Prophet said to the wife of Thaabit Ibn Qays "Will you return his garden?" She said, "Yes", then the Prophet ordered Thaabit to accept the garden and divorce her." [al-Bukhari]

Allaah knows best.