This is the most unusual rule in chess - make sure you learn it. En-passant occurs when a pawn has just moved two squares. It's possible the very next move for the opponent to capture that pawn with one of his or her own, as if that pawn had only moved one square. For example if a Black pawn on g7 advanced to g5 then a White pawn on h5 could capture it by moving to the g6 square. This is only possible on the very next move. If another move is played then en-passant is no longer possible.
