An orthotic is a custom made device that controls the amount of excessive pronation and supination that a foot goes through when walking, running or cycling. Runners will often say they are a pronator or a supinator. All people naturally pronate and supinate but excessive movement either way can be problematic. 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.
Supination is the opposite motion of pronation.
It allows the foot to be a more stable, rigid structure for when we push off on our next step.