The en passant capture is a reply to a double-square move by a pawn.
The fact that Pawns can move two squares on their first move might mean you miss out on a chance to take an opponent's Pawn.
A White Pawn on d5 might be hoping to take the Black Pawn on e7. But if it jumps to e5, then the White Pawn will miss out! To give your brave advanced Pawns a chance of taking opponent's Pawns, there is a special rule invented.
If an opponent's Pawn, which has not yet moved, tries to dodge past an advanced Pawn on the fifth rank by moving two squares, the advanced Pawn is allowed to pretend that the opponent's Pawn moved just one square. Like this:
[Event "en passant"]
[Site "DJCA"]
[Date "2023.??.??"]
[Round "1"]
[White "White"]
[Black "Black"]
[Result "*"]
[SetUp "1"]
[FEN "2k5/2p4p/8/3P4/5p2/8/6PP/6K1 b - - 0 1"]
[PlyCount "5"]
{[#]} 1... c5 2. dxc6 Kc7 3. g4 fxg3 *
You can do this only on the move after an opponent's Pawn has by-passed one of yours by moving two squares.
Leave it a move, and you lose the chance!