When you run, your heel hits the ground first. Your foot pronates by rolling inward and flattening out. Then your foot supinates by rolling through the ball and rotating outward. At this point it becomes rigid as it prepares to push off again. Many people overpronate and a very few people supinate.
Few people have perfect running styles.
Slight pronation is not a problem. The excessive pronator's feet roll inwards too much when running, which puts too much stress on the leg. The overpronator's arches flatten out when standing, although they may have a fairly high arch when there is no weight placed on it. Get your feet wet and walk on a piece of paper. If you can see the print of your entire foot including your arches, your feet probably over-pronate.