Room 101, introduced in the climax of the novel, 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.
You asked me once, what was in Room 101. I told you that you knew the answer already. Everyone knows it. The thing that is in Room 101 is the worst thing in the world.
— O'Brien, Part III, Chapter V