Plain English Meaning
It is how the pilot rolls the airplane left or right by turning the control wheel or moving the stick sideways. This tilts the wings, which is the first step in making a turn.
Full Definition
The pilot's use of the control wheel or stick to deflect the ailerons, which are hinged surfaces on the trailing edge of each wing near the tip. Moving the control left or right raises the aileron on one wing and lowers the aileron on the other, producing a rolling motion of the airplane around its longitudinal axis (the line running from nose to tail).
Context Anchor
Seen when learning how the airplane responds to control wheel or stick movement, especially during and when keeping the .