If the black pawn advances two squares on its first move and lands on d5, it has passed by White's e5 pawn.
White can capture this pawn as if it had moved only one square: the e5 pawn goes to d6 and the pawn on d5 is removed. This is called en passant. Try it!
Important: en passant is only allowed on the very next move. If you wait and play something else, the right is lost.
Now try it as Black: White has just pushed a pawn two squares from d2 to d4.