I lost because of it. It's when you move your pawn 2 ahead, to avoid a pawn on the 3rd rank. It really had me cursing. But I probably wont do it again.
En Passant

If your pawn is on its' original square and your opponents is 1 file either side, and you bring out your pawn 2 squares, he MAY take your pawn as if you moved only one square. He may only do this on the very next turn, not later.
Look at http://www.chess.com/learn-how-to-play-chess.html#special
for more info

En passant is a great move. It can come in very usefull at times. I personally like the rule. I also find it funny when I am accused of cheating because of the move.
It can also make your game a great deal harder when it is performed on you.

When it was used against me, I hadn't seen it before. I thought I was cheated! I thought the guy was a hacker! I thought he got in the system. I thought he knew the blackest of magic! Or Erik!
It can be very useful, yes.

Ha, yes En Passant is a very sneaky move. I played chess (casually) for a year before realising it existed! In my first tournament, against some 1000 and below players, I had just learned how to use the move. I got the chance to use it in a game, and I was really excited to play it. My opponent was baffled and looked up at me. He asked "What are you doing?", I said it was en passant and he didn't believe me, so we had go get the TD to come over to explain it to the poor guy. I gloated about how I got to use it in a game after the tournament was over. Later I learned that even though it is a flashy move, it is not always good.
Well, what is it?