Hyperkeratosis is one of the most common conditions a podiatrist treats. Hyperkeratosis is another name for corn or callus. They can show up anywhere on the feet and vary in seriousness from a small callus under the foot, to an infected ulcer that can appear under a corn on a toe.
Basically, the skin has hardened due to repeated friction and pressure. It is the body's way of protecting itself. Hyperkeratosis usually refers to a more scattered thickening of the skin (generally on the toes, but can show up on the metatarsal of the foot) whereas a corn is a thicker more centered area (more common on tops of the toes).