If there is not any pawn on a file, this file is called " Open File" .
As you see in the diagram No.1 d-file ( green one ) is an Open file. You have to take control of this file by using at least one of your Rooks and your Queen. It is better if you use more than one Rook and /or a Queen in order to occupy the open file.
Black controls now the open file with 2 Rooks and the Queen.