The rudder is the vertical control surface that controls the yaw of the aircraft.   Yaw is the left/right turning motion of the aircraft when viewed from above.   Yaw in the proper direction, toward the outside of the circle, is very important.   Without this yaw, the aircraft could turn toward the center of the circle, causing slack lines and loss of control.   By installing the rudder with an offset to the right the plane attempts to turn to the right and thus maintains good line tension and positive control.