In case anyone is wondering...

En passant is a little-known movement of pawns. All masters know it, but rarely set up for it because it's so niche and doesn't really do a heck of a lot.

https://en.wikipedia.org/wiki/En_passant#/media/File:Ajedrez_captura_al_paso_del_peon.png

It means "in passing" and applies because a pawn can take another pawn by not landing on the square it is on, as expected, but by "swiping" at it as it scoots by, landing a space ahead of the taken pawn.

It is one of two techniques, the other being Castling (where a King and Rook trade places granted the spaces between them are vacant), which makes it so hard for AI systems to turn Chess into a predictable Win/Stalemate certainty like Tic-Tac-Toe.

En Passant and Castling exponentially increase the possibility of moves and board compositions that are possible. Pawns, Kings, and Rooks can land in places they'd never be able to coordinate in novice play, if not for these two techniques. This exponential increase in possibility multiplies so fiercely the spectrum of potential moves the opponent will make, such that move calculations are crippled simply by the computation time required.

Most AI systems, no matter how robust, often crawl when confronted with clever and unexpected En Passants and Castling.