I see this follows your post on castling.
You might be interested in a collection of these rules, here: http://www.chess.com/learn-how-to-play-chess.html.
You'll find most people on this site are already familiar with the basic rules of chess, and some are even well-versed in deeper tactics and strategy. 
When a pawn advances two squares, if there is an opponent's pawn on an adjacent file next to its destination square, then the opponent's pawn can capture it and move to the square the pawn passed over, but only on the next move. For example, if the black pawn has just advanced two squares from f7 to f5, then the white pawn on e5 can take it via en passant on f6.