Truck Camera moves the camera parallel to the view plane.
Trucking a camera
For a target camera, dragging the mouse moves both the camera and its target parallel to the Camera view.
This button replaces the Pan button when a Camera viewport is active.
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:
To constrain trucking to a single axis:
The truck is constrained to the first axis you move while the Shift key is down.