Lose all your pieces (or get stalemated) to win the game.
Antichess (also known as the Losing Chess) is an eccentric variant where the objective is to lose all your pieces or to get stalemated (to have no legal moves when it's your turn).
Pieces move similar to as in standard chess, however with the exception that Kings lose their royal powers - they may not castle, and (walking into) checks are no longer a threat. Additionally, a Pawn may promote to a King. To compensate for the nature of Antichess, you will always be given a chance to decide for promotions, even if you have enabled the option to always promote to Queens.
Capturing is forced. When a legal capture is present for the player with the turn, they must take (to play any captures). This includes capturing en passant, and is what makes Antichess sophisticated.