Kuff is what is worn over the feet. It includes socks and could be made of various materials. God has permitted his servants to wipe over it during ablution if they cover the feet up to the ankles as a form of easiness. 

Wiping over the khuff

The Arabic word khuff refers to what is worn over one’s feet, whether it be in leather, wool or other material, including socks.[1]

Ruling and its basis

To wipe over the khuff instead of washing one’s feet for ablution is valid according to the general view of Sunni scholars. It is a concession given by Allah to make things easier for His servants. This concession is based on the Sunnah and unanimity of scholars. As for the Sunnah, there are numerous hadiths confirming its validity as the Prophet (peace be upon him) did it and instructed others to do it. Sunni scholars are unanimous that it is valid in travel and at home, whether there is a necessity or not.

Likewise, it is permissible to wipe over socks, because they are equally needed by people, and they are widely used. Hence, the permissibility to wipe over socks provided they cover the feet up to the ankles.[2]