A yaw damper is a device used on many aircraft (usually jets and turboprops) to damp (reduce) the rolling and yawing oscillations due to Dutch roll

Dutch roll

Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic modes...

mode. It involves yaw rate sensors and a processor that provides a signal to an actuator connected to the rudder. The use of the yaw damper helps to provide a better ride for passengers, and on some aircraft is a required piece of equipment to ensure that the aircraft stability remains within certification values.