An elevon is an aircraft control surface that combines the functionality of the elevator (used for pitch control) and the aileron (used for roll control), hence its name. It is frequently used in tailless aircraft such as flying wings.
There will be one or more elevons on each side of the aircraft at the trailing edge of the wing. When moved in the same direction (up or down) they will cause a pitching force to be applied to the airframe. When moved differentially, (one up, one down) they will cause a rolling force to be applied. These forces may be applied simultaneously by appropriate positioning of the elevons.