Room 101 is the most feared place in the Ministry of Love, where the Party's secret police, the Thought Police, interrogate and torture political prisoners. It is a place where the Party uses psychological torture to break down the prisoner's will and force them to betray their own beliefs.
The exact nature of the torture in Room 101 is never fully revealed, but it is implied to be something deeply personal and horrifying to each individual. The room is meant to break a person's spirit and force them to submit to the Party's authority.
The significance of Room 101 goes beyond its literal function. It symbolizes the ultimate power of the Party to control and manipulate its citizens, even their deepest thoughts and fears.