What is Room 101
By Part IV, Chapter 3, Winston has been arrested and indoctrinated mentally. Despite his long attempts to not surrender to the idea that two plus two is five, Winston now writes this very equation. Likewise, he records the other propaganda that he has ingested mentally. But, with a certain pride Winston acknowledges that although he has surrendered his mind, he has not relinquished his heart to the Party. In fact, he hopes that he can retain his hatred to the very end, and only release it in his final moments so that he can die with his loathing for the Party inviolate and thus defeat the Party in one way.
Not long after Winston ponders this small victory that he can win, O'Brien enters the cell and informs Winston that the final step must now be taken: Winston will be going into Room 101 of the Ministry of Truth, the room dreaded by all the Party's victims because they are subjected to their own worst terrors. This subjection of the victims is highly effective because they have no defense against their own irrational horrors. Fully cognizant of the power of this room, ironically named after initial college courses, O'Brien ushers Winston to a room in the bowels of the building. There Winston is strapped into a chair with a head restraint which pulls his head backward.
A sort of premonitory tremor, a fear of he was not certain what passes through Winston at his first glimpse.
Placed before Winston is a divided cage with his most dreaded objects: rats. Because pain has not been sufficient in removing Winston's love for Julia and his hate for Big Brother, O'Brien now threatens his prisoner with what he perceives as the worst thing in the world; then, he reminds Winston of his moments of panic occurring in his old dreams as he heard from the other side of a room "a roaring sound" in his ears. This terror is irresistible for Winston. He knows he can only transfer it to one person. "Do it to Julia!" he shouts in betrayal of his lover. Winston has failed the course taught in Room 101.