If you mean while in flight, this is rudder trim. On large airplanes, this is mostly controlled by the Yaw Damper, which automatically manages the yaw of the airplane by manipulating the rudder. On small airplanes this is done via a trim tab which is either bent to the correct angle or maybe be cockpit adjustable (like the one pictured below). This small tab causes the whole rudder to deflect the opposite direction, which in turn stabilizes and “coordinates” the airplane in level flight. During turns, the pilot would use the rudder pedals to “coordinate” (or account for adverse yaw). A rudder is rarely ever simply riding along in it’s vertical state.