What is the "En passant" rule in chess?
En Passant is when an opponent's pawn is two ranks away from your pawn row (either 4th or 5th rank depending on color), and you move your pawn two spaces forward with one move, so that is is next to his pawn.  You are allowed to make your first move with a pawn two spaces, but the En Passant capture allows your opponent to capture you just as if you had moved one space.  Note, that this capture is only allowed as the very next move.  This prevents you from being able to sneak pawns by your opponent's pawns.



It's not only useful, but chess would be a completely different game without it, since everyone would have to be very concerned about moving a pawn to the 4th or 5th rank, knowing that a pawn could get past them.  The main way you could change it so that the game wouldn't change radically is to disallow 2-space moves by pawns, but that is still a substantial change from the way's it been*, and would eliminate the majority of standard openings. A less tumultuous change would be to simply prohibit double movement in the cases where an en passant capture would be possible. This would have almost the same exact effect as the en passant rule, since it happens more often that the threat of an en passant capture is enough to make an opponent not move two spaces, and actual uses of the move are rare.

* unless you go back to the 15th century, when the ability to move pawns two spaces was added to the game.