When you think of a game you really want to make you should be able to explain the main idea within one paragraph. That paragraph is the focus of the game, a reference point for the game to be designed from. The focus should help determine what direction should be taken when a design decision comes up. The game's concept should be explained without comparing it to another specific game or series and it should not go into detail on how its gameplay works. The focus breaks down the game idea to the core feel and concept. Sharing it with other developers before and while making the game will unite the team under a shared vision.