A prop is a game aid that is intended to represent something in the game world, often so that the players can interact with the prop in a similar way to their player characters might interact with the item in the game world. The name is derived from props as used in film and theatre.
A common form of prop is a mock document, which is cheap and easy to prepare. A GM can produce a mock document with just some pen and paper, although some may produce something more extravagant. Mock documents allow players to receive new information by the same medium as their characters (e.g. if a character receives and reads a letter), rather than being told of a document's contents by the GM (or fellow player). Mock documents can also, like notes, be shared with only specific players, rather than the whole group.