To unload is to remove cargo from a vehicle or shipping carton. Unload can also mean to discharge or get rid of something, or to get something off your chest.
A clothing store might unload (remove) a hundred cartons from the delivery truck, and then unload (unpack) shirts and pants from the cartons. If a carton of clothes is imperfect, the store might send them back or unload (dispossess themselves of) them by selling them cheaply to a factory-seconds outlet. Then the store manager, depending on her mood, might unload on (give a tongue-lashing to) the manufacturer for shipping shoddy products.
