When a pawn advances two squares from its original square and ends the turn adjacent to a pawn of the opponent's on the same rank, it may be captured by that pawn of the opponent's, as if it had moved only one square forward. This capture is only legal on the opponent's next move immediately following the first pawn's advance.

For example: if the white pawn moves from e2 to e4...

...the black pawn on f4 can capture it en passant, moving from f4 to e3...

...while the white pawn on e4 is removed from the board.

This is the only move in chess where the capturing piece ends up to a different square than where the captured game piece was. En passant was added at the same time with the two-square advance opening move. It prevents a pawn from using the two-square advance to pass an adjacent enemy pawn without the risk of being captured.