En passant is a special pawn capture: when an enemy pawn advances two squares and lands beside your pawn, you may capture it as if it had moved only one square. The capture is available only on the very next move — if you delay, you lose the right. Your pawn moves diagonally into the square the enemy pawn skipped over, and the enemy pawn is removed. This rule exists so a two-square first move cannot slip safely past a waiting pawn. It is one of the most commonly missed rules by beginners, so watch for it whenever an enemy pawn rushes forward.