Multiplayer means, several players taking part at once. In Football Manager / Championship Manager there are various methods of playing a multiplayer game: Hot-seat and Network play.
This is the most common and simplest type of multiplayer game, where several people share the same computer and take it in turns to manage their team. In recent editions of FM, there is no limit to the amount of human managers that can be added to a game.
This is where 2 or more computers are connected either by a LAN (local area network) or through the Internet. This way is more difficult to set up that a hot-seat multiplayer game but it means that all managers can play at once rather than waiting for their turn.