Every pawn, on its first move, has the choice of moving either one or two squares
Look at this diagram.
Black may think, "If I move my pawn one square White will capture it so I'll move it two squares." and the position would be as shown in the diagram below:
However, White can still capture the black pawn as if it had moved only one square.
The new position would be as shown in the diagram below. White removes the black pawn from the board and places the white pawn on the square the black pawn would have moved to if it had only moved one square forward.
This special way of capturing is called capturing en passant and is abbreviated e.p. En passant is a French expression which means "in passing" The en passant capture must be done immediately after the black pawn advances two squares. If White plays another move he cannot then decide to capture en passant!