The foot is one of the most complex mechanisms in the entire human bone structure and the first line of defense against ground forces when running or walking.  Pronation (basically the inward movement of the foot toward the big toe) is the body's natural form of shock absorbtion.  As the bones of the foot rotate angularly inward, ground forces are distributed more evenly about the foot reducing those forces up through the body.  There are 2 movement patterns that disrupt the natural process of shock absorbtion through pronation.  They are known as "over pronation" and "suppination".  "Over pronation" (common in but not limited to people with flat feet) is dangerous because the foot rotates too far inward and as the foot levers itself from the ground it has not been given ample time return to a safe rigid platform from which to push off.  This transfers the levering forces caused by the impact of the ground from the muscles designed to bear those loads to other areas of the body not intended for that purpose.  It also cause an imbalance through ankle, knee, and hip joints again putting pressure on areas not designed to bear those loads.  the opposite of over pronation is suppination (common but not limited to people with high arches) and that is the outward rotation (toward the pinky toe) of the foot.   As stated before, normal pronation is the body's way of absorbing shock caused by ground forces.  Suppination does not allow allow the foot to rotate inward and absorb that shock so the brunt of the forces are felt in the bones to the outside of the foot and are not dampened as they up the body.