En Passant is a french expression that means 'in passing'. It is a special type of capture that pawns can make under very specific circumstances. It's not a very difficult rule to understand, but many people never learn it and may accuse you of cheating when your pawn suddenly makes a move they weren't expecting! However, if you want to improve at chess, and especially if you want to play in leagues and tournaments, then it's vital that you know all the rules of chess and not just the easy ones!
Take a look at the board below.
If the black pawn moves one square, then the white pawn can capture it on the next move. So, black may decide to move it two squares instead, to try and rush past white's pawn without being taken. Then the board would look like this:
However, en passant is a special rule designed to stop players from being able to sneak their pawns past like this. White is able to capture the pawn as if it had only moved one square. The white pawn moves diagonally forward as if the enemy pawn had only moved one square, and the black pawn is removed from the board. See the result below:
Use the arrows on the board below to see this in action:
In order to do an en passant capture, your pawn must be on the fifth rank from your side of the board (we refer to horizontal rows as 'ranks' in chess), the enemy pawn must be on an adjacent file (a 'file' is what we call a vertical column), and it must move two squares in one go. Then, on the very next move, you can capture the enemy pawn en passant, but if you don't do it immediately, then you lose the right.
On the following board, en passant is not possible because the white pawn is on the sixth rank. Your pawn must be on the fifth rank only in order to capture en passant.
On the board below, en passant is not possible either, because the black pawn has only moved one square.