Answer:
In 1984. room 101 was the place, inside the Ministry of Love, were prisoners were taken to face their worst fear. This was done with the goal of punishing the prisoners, but also with the goal of changing their ideas.
In the case of the protagonist, Winston Smith, his worst fear is rats. For this reason, when taken to room 101, Winston's head is put inside a cage, where there are rats inside. Winston, faced with the prospect of having his face eaten off by the rats, finally succumbs and changes his mind on Big Brother: he becomes a Big Brother follower, just like everyone else.