A double check is the most forcing move in chess. You make a double check by using a discovered check and also checking with the piece that you move. The only way out of a double check is to move the king, so often a double check is checkmate!
White can use a double to win the game, can you see it?
By moving the knight the king is checked by the rook, and the bishop, leaving no moves left for the king to deal with two checks.