Landing gear is the pair of crank-down legs beneath the front of a trailer. When the trailer is disconnected, the driver lowers them to support its weight; to hook up, they raise them back up so the trailer can roll.
Setting the landing gear correctly is essential during a drop and hook: if it’s not fully down or on solid ground, a parked trailer can tip or drop. It’s a small step drivers do constantly, and getting it wrong can damage the trailer or its freight.