En passant is a special chess rule that allows a pawn to capture an adjacent enemy pawn that has just advanced two squares from its starting position, as if it had only moved one square.
This capture must be made immediately on the very next move — it cannot be delayed. In practice: if your White pawn stands on the 5th rank, and the opponent’s neighboring pawn advances two squares to land beside yours, you may capture it diagonally, placing your pawn on the 6th-rank square the enemy pawn skipped over. The rule is symmetrical: a Black pawn on the 4th rank may capture the same way when a neighboring White pawn has just advanced two squares. Wait even one move, and the right to capture en passant is permanently lost.
Whenever your pawn reaches the 5th rank (White) or 4th rank (Black), make it a habit to verify whether an en passant capture is available before committing to another move. Equally, think ahead before advancing your own pawn two squares — your opponent may be waiting to use this rule against you.