En passant is lesson 3 of 3 in Castling, promotion and en passant, a basics course suited to players rated around 350 (complete beginners). En passant (French for 'in passing') is a special pawn capture. It has 2 interactive exercises: 2 find-the-move puzzles. You play every move on a board and get feedback straight away. The lesson is free.
What this lesson explains
Black's pawn just jumped two squares, from d7 to d5, and landed right next to your pawn on e5. Your pawn may capture it as if it had moved only one square: it moves to d6 and the pawn on d5 is removed. You must do it on the very next move or never.
What you practise
Black just played d7 to d5. Capture it en passant!
Black's pawn rushed past yours from f7 to f5. Take it en passant.
chessGM