In simple term, pronation is the flattening out of the arch when the foot strikes the ground. Normally, the foot will pronate to absorb shock when the heel hits the ground, and to assist in balance during mid-stance. The ankle will 'tip' towards the inside. The excessive stress on the The body can over compensate for this pronation and shift the ankle towards the outside causing the ankle to roll over.
Supination is the opposite motion of pronation.
Supination allows the foot to be a more stable, rigid structure for when we push off on our next step. It does in not take much force to cause the ankle to roll over, potentially causing ligament damage. It is the body's way to absorb shock and allow the foot to work as a lever. Excessive motion in either direction can be very problematic if not controlled.