Plain English Meaning
How far the rudder is moved left or right from its straight-back position when the pilot presses a rudder pedal. The more the rudder is moved, the stronger the sideways push on the tail that swings the nose left or right.
Full Definition
The angular movement of the rudder away from its neutral (streamlined) position, produced by pilot input on the rudder pedals. Rudder deflection creates a sideways aerodynamic force on the vertical tail, which yaws the airplane around its vertical axis. The amount of deflection is limited by mechanical stops and, at low airspeeds, by how much pedal the pilot can apply.