The Battle Room is both a location and a game at the Battle School.
As a Location
Although the official reason for the Battle Room being at the centre of the school is that it is exempt from the centrifugal force that provides gravity to it, the actual reason is that there are several Battle Rooms that rotate on a schedule. It is later mentioned in Ender’s Game that many of the technologies (likely including generated gravity) are generated at the school by adapted Bugger technology.
As a Game
The game played in the Battle Room is a simulation of interstellar skirmishes that familiarize the students with the concepts of zero gravity movement and tactics. Each participant wears their own Flash Suit in the colour of their army and are equipped with a Flash Pistol to disable their opponents. The goal of the game is to disable the opposing army using their flash pistols and to unlock the enemy gate using four soldiers, with a fifth member passing the threshold to win. Obstacles are provided via Stars, floating bodies of mass that are meant to represent challenges in a fight, such as asteroids or disabled starships, for example.
Each army’s commander is responsible for scheduling practices in the Battle Room, overall strategy and improving the performance of their army. The skirmishes are different in their victory conditions and star layout.