A flaperon is an aircraft wing lateral control surface. The idea behind a flaperon is that if you decrease the lift on one wing and maintain it on the other, the plane will roll. Technically, they are a mix between ailerons and flaps, hence flaperons.
The actual flaperon is a moveable surface at the trailing edge of the wing, extending into the airstream whenever the pilot needs reduced lift on the wing surface.
Flaperons came into use as aircraft speeds increased, giving the pilot greater control of the aircraft than he or she would have with just ailerons. | 
