“En passant” (French for “in passing”) is a special pawn-capturing rule. If your opponent pushes a pawn two squares forward from its starting line to land right next to your pawn, they haven’t safely escaped. For one turn only, your pawn can capture their pawn diagonally as if it had only moved forward one single square. It is the only move in chess where the capturing piece does not actually land on the same square as the piece it removes.
For experienced players
The en-passant rule was added to chess relatively late in history to prevent pawns from using their new two-square initial leap to unfairly bypass enemy blockades.
See also:How the Pawn Moves