It is a special pawn capture that can only occur immediately after a pawn moves two ranks forward from its starting square and an enemy pawn that could have captured it had it only moved forward only one square. The opponent captures the just-moved pawn "as it passes" through the first square. The resulting position is the same as if the pawn had moved only one square forward and the enemy pawn had captured it normally.
It is a special pawn capture that can only occur immediately after a pawn moves two ranks forward from its starting square and an enemy pawn that could have captured it had it only moved forward only one square. The opponent captures the just-moved pawn "as it passes" through the first square. The resulting position is the same as if the pawn had moved only one square forward and the enemy pawn had captured it normally.