Yaw damper is a gyro system which acts on the rudder(s), this for aircraft stability.
Have you ever heard about Dutch roll ??? (nothing against our friends from NL
Some (swept wing) airplanes are unstable on the yaw axis.
This because if they yaw to the left, the RH
(swept) wing will present itself as a straight wing.
Straight wings create more lift, and more drag...
More lift = that RH
wing will go up - More drag = RH
wing is pulled aft (around yaw axis)...
Result, now, the RH
wing being further aft, the LH
wing will do that game...
wing will get more lift, more drag... result is the aircraft will yaw L and R.
Result could be a loss of control = Dutch roll.
So the yaw damper prevents Dutch roll.
Generally, the yaw damper is the "3rd axis" of the autopilot.
Modern airplanes generally have a full time "series" yaw damper, engaged at all times.
Some airplanes being unstable, their yaw damper is a "NO GO item" on the MEL
Some other airplanes may be limited in speed and altitude if yaw damper is INOP.
The 747s have 2 yaw dampers... But the airplane flies perfectly without them. Not a NO GO item for 747s.
Hope this helps you...