En passant ("in passing") is the most misunderstood rule in chess, and the source of a thousand "this site is broken!" complaints that are actually legal moves. When an enemy pawn advances two squares and lands directly beside your pawn, you may capture it as if it had only moved one square, but only on the very next move. Wait a turn and the right is gone forever.
Why does the rule exist? The two-square first move was added centuries ago to speed up openings, and en passant was added with it so a pawn couldn't use that speed boost to sneak PAST an enemy pawn that was waiting to capture it. It keeps the pawn structure honest.
Black just pushed the d-pawn two squares, from d7 to d5, landing right beside White's pawn on e5.
Black just played d7-d5. Capture the pawn.