When you take a step, your foot typically hits the ground heel first and rolls toward your toes, flattening the arch slightly. As you push off the ball of your foot, your arch springs back and does not touch the ground. That's how normal feet are supposed to work. Unfortunately, many feet aren't normal.
Overpronation occurs if your foot rolls too much toward the inside. This can cause arch strain and pain on the inside of the knee. Underpronation occurs if your foot rolls too much to the outside. Underpronation can lead to ankle sprains and stress fractures. You can relieve foot pain by compensating for these tendencies, but first you need to determine which way your feet roll.
One method for determining which kind of pronation you have is the watermark test: Put your feet into a bucket of water, then make footprints on a piece of dark paper