These are games where a player effected checkmate by capturing a pawn en passant. It's very rare, I think even rarer than mating by castling. I think it's become more common in recent years since anyone can play thousands of games on the Internet. I have the rare distinction of having achieved this feat twice! IM and chess streamer Eric Rosen, who has surely played more games than I, says that he has also done it twice, which I guess makes us the unofficial joint world record-holders.

Eric looks at 25(!) games with en passant checkmates, assembled for him by Mark-Dev, who wrote computer script to find such games from games played on lichess, at https://www.youtube.com/watch?v=qKX... Mark-Dev's report, and all of those games, are at https://github.com/mark-dev/chessfa... If you're looking for moves that are even rarer than en passant mates, see https://www.youtube.com/watch?v=iDn...