What does en passant mean in chess?

Answer 1

Pawns can move one space forward each turn except for the first time you move a pawn. The first time you move a pawn (for that particular game) you are permitted to move the pawn two spaces forward.
En passant is a special way for a pawn to capture another pawn. Say a white pawn moves two spaces (on its first turn) in such a way that it is directly next to a black pawn. Without en passant, black could not capture this white pawn, since the white pawn passed over the space which the black pawn could attack.
The black pawn can now move diagonally behind the white pawn and capture it. This is called en passant.

Answer 2

1 A player moves his pawn two squares, from his second row to his fourth row.
2There is a pawn of the opponent that can capture at the squares that is passed over by the pawn.
3 In that case, this pawn of the opponent has the right to capture en passant in the directly following move.
4 To capture en passant, the opponents pawn goes to the square passed over by the pawn (i.e., the square on the third row), thus moving diagonally forwards. The captured pawn is taken from the board.

Answer 3

"En passant" comes from the French for "While passing". It only applies in a special situation. Say you have a pawn in your second (starting) row and your opponent has a pawn in his fifth row (your fourth row) and one rank away from your pawn. If you move your pawn out only one square his pawn would be able to capture yours. So you move it out two squares to avoid capture, since then the pawns will be in the same row.

The En Passant rule states that your opponents pawn can then "interrupt" your last move and capture your pawn in passing as if you only moved it out one square. He has to do this the very next turn or else won't have this option on subsequent turns.