Room 101 is a torture chamber in which a prisoner is subjected to his worst nightmare. Such is the omniscience of the state in the totalitarian society of 1984 that even a citizen's nightmares are known to the authorities. The nightmare - and therefore the threatened punishment - of the protagonist Winston Smith is to have his face gnawed by rats. Smith saves himself by begging the authoritites to let his girlfriend, Julia, (he is married but has not seen his wife in years) have her face gnawed out by the ferocious rodents.
Room 101 is supposedly named after a conference room at BBC Broadcasting House where Orwell used to sit through tedious meetings.
Room 101, like Floor 13, is often used in popular culture to give a sinister meaning to what would otherwise be an unremarkable room. For example, Thomas Anderson (aka Neo) lives in Room 101 at the beginning of the 1999 movie The Matrix.
For information about the TV series of the same name, see Room 101 (television).