Payload is what a vehicle carries. If you have a plane with a payload of one ton, then that plane can carry one ton (including you and the snacks you may bring aboard).
Often, payload is estimated to be everything on board a vehicle that's worth money, or that produces income for the vehicle's owner. In the case of a commercial jet, that might be all the paying passengers. In other cases, a truck, ship, or plane's payload includes every single person and item on board, including the flight crew and fuel. From about 1936, payload frequently referred to bombs carried by a military plane or missile.