It's a multiplayer game where most of the players are astronauts with a set of tasks to complete. Some players are assigned to be imposters, who look just like the astronauts but whose goal is to kill the other astronauts. The astronauts win by completing the goals or identifying and killing the imposters based on behavioral cues. The imposters win by killing the astronauts before they can complete the tasks. Incidentally it's the game that spawned the term 'sus', which suggested a player was suspicious that another player was an imposter