Room 101, introduced in the climax of the novel "1984", is the basement torture chamber in the Ministry of Love, in which the Party attempts to subject a prisoner to his or her own worst nightmare, fear or phobia, with the object of breaking down their resistance.

Such is the purported omniscience of the state in the society of Nineteen Eighty-Four that even a citizen's nightmares are known to the Party. The nightmare, and therefore the threatened punishment, of the protagonist Winston Smith is to be attacked by rats. This is manifested in Room 101 by confronting Smith with a wire cage that contains two large rats. The front of the cage is shaped so that it can fit over a person's face. A trap-door is then opened, allowing the rats to devour the victim's face. This cage is fitted over Smith's face, but he saves himself by begging the authorities to let his lover, Julia, suffer this torture instead of him.

The threatened torture, and what Winston does to escape it, breaks his last promise to himself and to Julia: never to betray her. The book suggests that Julia is likewise subjected to her own worst fear (although it is not revealed what that fear is), and when she and Winston later meet in a park, he notices a scar on her forehead. The intent of threatening Winston with the rats was to force him into betraying the only person he loved and therefore to break his spirit.

More Info: en.wikipedia.org