The rudder is a control surface that allows changes to be made to the attitude of an airplane on the yaw axis. The rudder is usually a movable flap on the trailing edge of the vertical stabilizer at the tail of an airplane. In that configuration, when the rudder is moved to the left, it increases the lift of the vertical stabilizer causing the tail of the airplane to move to the right, which in turn causes the nose to move to the left. The reverse is true when the rudder is moved to the right.
The rudder is used to control turns in non-aileron RC trainer airplanes. The rudder initiates the turn, and the additional lift generated by one side of the wing due to dihedral causes the airplane to roll and continue the turn.