The rule closes a loophole created by faster pawns
En passant—French for “in passing”—makes sense once you see the problem it was designed to solve. In the late medieval period, chess rules changed to let a pawn move two squares on its first move. That sped up the opening, but it also let a pawn slip past an enemy pawn that would have been able to capture it had it advanced only one square at a time.
The new capture rule kept the two-square move from granting that special escape. It treats the advancing pawn as though it had moved just one square, allowing the adjacent enemy pawn to take it on the square it passed over.
The opportunity lasts one move
Suppose a white pawn sits on its fifth rank and a black pawn advances two squares from its starting position, landing beside it. White may capture en passant: the white pawn moves diagonally to the square the black pawn crossed, and the black pawn is removed from the board.
The capture is optional, and it must happen immediately. If White makes any other move, the opportunity disappears. That limit keeps the rule tied to the two-square advance rather than turning it into a lingering special power.
A small exception with a clear purpose
En passant can look like a pawn capturing empty space, but the target square is not really empty in the logic of the rule: the opposing pawn has just crossed it. The move preserves the ordinary risk of advancing beside an enemy pawn while still allowing first-move pawns to cover ground quickly. So the odd-looking exception is the price of making the faster opening move without letting it dodge a capture.
