Both 足 and 腳 mean foot.
腳 is used in modern day spoken and written Chinese to mean foot, such as 他的腳很大
足 on the other hand is used to mean foot in idioms, such as 畫蛇添足, in Classical Chinese to mean foot or a foot like thing that supports an object, such as 吳之势强,鼎足之形成矣, and in foot related vocabulary, such as 足弓.
Laurence