Yaw has two main meanings in MWLL. Both meanings typically involve using the [mouse] X-axis, left and right. Movement along the mouse's Y-axis is called pitch. Yaw is sometimes called twisting, which itself has multiple definitions including spreading out damage.
As a Maximum Twist
Most Mechs and some tanks like the Long Tom are limited by how much their turret or torso can twist or rotate. The maximum twist can be found on each asset's wiki page under Torso yaw and Turret rotation.
As a Maneuvering Direction
VTOLs and Aerospace both aim and maneuver left and right using yaw. By aiming left and right with the [mouse] X-axis, aircraft can accurately target enemies. For making sharp turns, aircraft should use roll and pitch instead of yaw.
Aircraft have no maximum yaw limitations like some tanks or most mechs.
For additional information on how to use yaw to maneuver an aircraft see the Air Combat Guide.