Payload is any card that functions as the purpose of a deck, most often in engines.
 In Engines
An engine needs three parts: +Action (normally through villages), draw, and payload. The +Action is necessary to play everything, and the draw is necessary to get everything you want to play into your hand. The payload is the reason you built the engine in the first place; a deck that only has +Action and draw will usually be outpaced by big money. The most common payload is +Buy; simply adding the ability to buy more than one card per turn can make a Village/Smithy deck viable. Equally useful are terminal silvers (and other terminal + cards), which give you the buying power to make use of extra buys, and trashers, which trim your deck down and make your engine run more smoothly. Other payload categories are gainers, cost-reducers, and Attacks; Treasures can also function as payload.
Some villages and draw cards have additional abilities that can function as payload; a Fishing Village/Wharf deck does not need any other cards to win. Outside of these situations, most payload cards are stop cards.