In this diagram, White has a pawn is on rank 5 (row 5). If Black moves the pawn on b7 two squares to b5, the en passant rule allows White’s pawn on c5 to capture it as if Black had moved the pawn only one square. The Black pawn on b5 is removed, and the White pawn is placed on b6.

The en passant rule stops Black's pawn "sneaking past" White's b5 pawn.

The en passant capture must be done straight away, on White's next move following Black’s pawn move two squares forward.

(En passant is the French for “in passing”).

Another description of en passant

Suppose that I am White and you are Black. Suppose also that, during the course of the game, one of my pawns ends up three squares ahead of its start position. This is just past the half way down the board (figure 1).

Take a look at the files (columns) just to the left and right of my pawn. If you move a pawn on one of these two files from its start position two squares forward, it would sit on the square next to mine (figure 2).

In en passant, I am allowed to move diagonally to the space behind your pawn (to the spot you would have occupied had you only moved one space forward), and capture the pawn you have just moved (figure 3).

I am only allowed to do this in the move immediately after your pawn move.