One of the most confusing and overlooked rules of chess is the En Passant (in passing) rule. Although you may play a hundred games, and never have to use the En Passant rule, you should know it if your opponent uses it.
In the diagram below, White's pawn is on the fifth rank (horizontal row). The En Passant rule is designed to protect White from Black's pawn "sneaking past" White's pawn by jumping forward two squares (from b7 to b5). If you don't remember that a pawn can move twice on their initial move only, refer back to Pawn Moves. Under the En Passant rule, if Black moves the pawn two squares to b5, the White pawn can take it by moving to b6 (just as if Black had moved the pawn one square). This must be done on White's next move. He captures the pawn En Passant, or in passing. The Black pawn on b5 is removed, and White moves its pawn to b6.
Note: When given an opportunity to capture a piece En Passant, do not always take it. Too many novice players jump on the chance to take a pawn En Passant, and do not consider the situation. As always, look at all your options.