Rules :: What is en passant?
En passant is a move where a pawn can take another pawn, under strict condition, not obeying the general rules of pawn captures.
In this sequence, as shown in the diagrams, the white pawn can take the black one, not taking in diagonal. However this only works, if the black pawn moves two squares. White has to take the pawn on his next move, or then it doesnt work anymore.
This is a position from a real game, where black moves his pawn two squares, and white can take it.