Actualy I'm 90% certain it refers to the game Dungeons & Dargons as played with pen and paper, which often takes place in Forgotten Realms.
My bad, I guess to be more exact:
Q: What is D&D (Dungeons and Dragons)?
A: D&D is many different things to many different people, but at its root it is a P&P (pencil and paper) game. It is based upon roleplaying and war gaming, and incorporates aspects of both worlds - so there is no "right" way to play D&D.
Q. What are the Forgotten Realms?
A. The Realms are a setting for the D&D game. They started out as a group of regions in a single continent, and have steadily grown till they are a complete and complex world. Extra supplement books have added to the original setting, and the Forgotten Realms Novels have helped the world become more rich and real.