Truck Light moves a target light and its target parallel to the light view, and moves a free light along its XY axis.
Trucking a light
To constrain trucking of any viewport to a single axis, hold down the Shift key. The truck is constrained to the first axis you move while the Shift key is down.
To accelerate trucking, hold down the Ctrl key.
The camera and its target move parallel to the view plane, which is perpendicular to the camera's line of sight.
To truck with the middle mouse button: