I copied and pasted this from a link. It doesn't give the whole information but here it is.
There are two terms that describe the motions a foot goes through when walking and running: pronation and supination.
Pronation is the inward roll of the foot that allows the body to absorb shock and adapt to uneven surfaces. It occurs as soon as the heel touches the ground.
Supination is the slight outward roll which makes the foot more rigid and ready for push-off. It occurs as your body weight moves over your foot from the midfoot to the forefoot.
Injuries can occur when one's foot either pronates or supinates excessively. Those who are predisposed to excessive pronation strike the ground on the outside portion of their heel to the extent that their foot continues to roll in past the point where it should stop. This flattens the arch of the foot. The result - despite the fact that excessive pronators absorb shock well - is instability in the foot. An unstable foot can and will cause shin pain, knee pain and low back pain.
Those with excessive supination have the opposite problem. They often have inflexible feet with high arches. While such feet are stable, they don't absorb shock well. As a result, runners with this problem often suffer injuries that result from jarring forces (like stress fractures).
Shoes can dramatically affect foot mechanics. Therefore, wearing the correct shoe can help prevent injuries; conversely, wearing the wrong shoe can increase the odds of incurring foot problems.
Here's the picture to give you an idea what overpronators/underpronator/normal looks like