En passant is a rule allowing a pawn to make a special-case capture.
En passant means "in passing." This strange move was added to prevent players from creating passed pawns by using the pawn's 2-square first move to sneak past defending pawns.
En passant is a special type of pawn capture allowed only when one player moves a pawn two squares forward and in doing so, ends up being adjacent to an opponent's pawn. On the next move only, the second player has the option to capture the pawn en passant by capturing as if the first player's pawn had only been moved one square. It literally moves diagonally onto an empty square. The pawn previously moved two squares is captured.