A GM screen is usually something the GM uses to keep things away from players. These can be rolls, or the notes that the GM may be using for the game. It's the stop the players seeing what the GM has planned.
Usually, GM screens have a mighty selection of tables and charts that the GM may need to use so he doesn't have to look through the book every time the event comes up. On the opposite side it has pretty pictures for the players to look at.