You can 'En Passant' when one of the opponent's pawns moves forward two spaces on one move, to directly be adjacent to a pawn of your own on it's left or right. You can then 'En Passant' by putting your own pawn behind the opponent's pawn that had just moved.
For example, you have a pawn at H4, and the opponent's pawn at G2 moves forward to G4. You would be able to use your pawn from H4 to G3, and take the opponent's pawn which is at G4.
This achievement can only be done in Classic mode.