What is En Passant ?

What is En Passant. En Passant is capturing a pawn next to it as though it went one step. Let me show you an example. 

That is En Passant. You can only En Passant when your opponent's pawn moves 2 step forward next to yours. It is actually a one time offer. Let me show you what I meant

When you move another piece, you won't be able to En Passant.