En passant, French for "in passing," is one of the most frequently misunderstood rules in chess. It can only happen under specific conditions: a pawn must advance two squares from its starting rank, and an enemy pawn must be on an adjacent file on the fifth rank (for White) or fourth rank (for Black). The capturing pawn moves diagonally to the square the opponent's pawn passed through.
This rule exists to prevent pawns from using the two-square advance to bypass an opponent's pawn that has reached the fifth rank. Without en passant, a pawn could avoid confrontation by leaping past an enemy pawn, which would fundamentally change pawn structure dynamics.
En passant must be exercised immediately on the next move or the right to capture is lost. In practice, en passant captures are relatively rare but can be tactically important, sometimes opening files, creating passed pawns, or delivering discovered attacks.