This is no ordinary FPS. In this game, the player and the enemies take turns acting instead of going all at once. When it's your turn, your actions are controlled by how many action points you have; when you use all your action points, the turn then moves to one of the enemies, who then act their turns before it comes back to you.
You win by killing all the enemies and not dying. If they kill you first, you lose. Easy to understand, hard to execute.
