En passant is a French word. The rule is explained here. If a white pawn is in the fifth rank and one of the opponent’s black pawns moves from the seventh rank to the fifth rank from the side file of the white’s 5th rank pawn can capture the black pawn in the very next move. And the white pawn will move to the captured black pawn’s file on 6th rank.
In the same way, when any black pawn stays on 4th rank at this time from its sides file any white pawn moves 2nd rank to 4th rank then the black pawn can capture this white pawn in the very next move and the black pawn will move to this capturing pawn’s file on 3rd rank.
Here if black moves his pawn d7 to d5 then white can capture the black pawn and the white pawn will move from c5 to d6.
En passant only happens at the time of the very next move not possible at other times.
