It's an "en passant" problem. Black's last move had to have been 1..., e5. Then when white captures the pawn en passant with 2.dxe6 e.p., black is mated by the white bishop on b2 since tbe black f7 pawn is pinned by the white b7 rook and can't interpose to block the bishop.

It's an "en passant" problem.
Black's last move had to have been 0..., e5. Then when white captures the pawn en passant with 1. dxe6 e.p., black is mated by the white bishop on b2 since the black f7 pawn is pinned by the white b7 rook and can't interpose to block the bishop.