Your foot is like your leg's hand, and a foot is twelve inches. When you walk, you go by foot.
Since feet are so important, the word foot has many meanings. The main one is that thing below your ankle — people have feet, and so do many animals. The lower part of a bed is the foot of the bed — the foot of a mountain is also where you would expect it. The lower part of anything could be called a foot. Paying for something is footing the bill, and footing up a column of numbers is adding them up.