In chess, there are three special moves: castling, pawn promotion and en passant. The first two are quite well known, but the third is less recognised by amateurs and is more of a “secret move” for more experienced players. Thus, many beginners complain their opponent is cheating, when they are using a perfectly legal move.
En passant is French for in passing - the etymology becomes clear once one understands how the move works.
Although a pawn can usually only move one space forwards, it can move up to two spaces on its first move. En passant only applies to a pawn that has moved two spaces. For example, if a white pawn moves two spaces forward and a black pawn is positioned to its left or right, the black pawn can move diagonally behind the white pawn to take it. This is because if the white pawn had moved one space, it would have been in the normal attacking range of the black pawn. Ergo, en passant is a technique that can stop a pawn from penetrating the defensive line and charging forwards.
This move must be used the turn after the pawn moves two spaces. Otherwise, the right to en passant disappears (i.e. cannot wait a turn to use it). In chess, this is the only move where the attacking piece lands on a space other than the taken piece.
Why was the en passant created? The reason being, the two-space first move rule came into place around then, so the en passant was devised to balance it, while complementing the pawn’s short attack range and inability to move backwards.