A custom orthotic is a device designed to align the foot and ankle into the most anatomically efficient position. They look like insoles, but are biomechanical medical appliances that are custom made to correct your specific foot imbalance.
Custom orthotics work on your feet much like glasses work on your eyes - they reduce stress and strain on your body by bringing your feet back into proper alignment. The plastic body of the custom orthotic helps to re-align the foot by redirecting and reducing certain motion that takes place during the gait cycle.
Custom orthotics fit into your shoes as comfortably as an insole - and they have the advantage of having been made from precise imprints of your feet.