A mod, or modification, is something that changes gameplay in some way, if that's what you were asking. Or a moderator on a multiplayer game there to find rule breakers. An example of a modification to a game is the Rei's Minimap mod to MineCraft, which adds a minimap
It's something that changes or adds something to the game. For example, a house mod is a mod that adds or changes a house in the game, and a weapon mod is a mod that adds or changes weapons in the game.