A lift gate is an add-on piece of equipment for the back of a truck that allows cargo to be moved to ground level.
Standard trucks require a loading dock above the ground in order to properly unload. This dock is 48-52 inches above the ground and often has plates and other equipment to assist in unloading. If you're trying to move pallet items or other heavy items from the height of the truck bed to the ground instead, you'll use a lift gate. The gate unfolds to create a small platform, which you roll the pallet onto. Then you lower the platform to the ground to unload.
Lift gate deliveries are essential if you're not in a location built for industrial use. If you're scaling up a production business out of your kitchen and need to have a large shipment delivered, you'll need to let the supplier/carrier know that you need a lift gate. There is often an added fee of $100 or so added to the delivery cost when this service is required.