top of the foot; instep
Refers to the upper surface of the foot, from the toes to the ankle. The medical term 'dorsum of the foot' is a direct equivalent.
The new shoes press against the top of my foot and hurt.
I broke a bone in the top of my foot.
Compound of 足 (foot) + の (possessive particle) + 甲 (upper surface, instep). The word 甲 originally referred to the hard outer shell of a crab or turtle, and by extension the upper surface of the foot.