What is in Room 101, as O'Brien describes it?

At the end of
,askswhat is in Room 101, to which he replies:

You know
what is in Room 101, Winston. Everyone knows what is in Room 101.

This foreshadows Winston's time in Room 101 in Part Three, Chapter Five. Here, O'Brien
elaborates by saying that the room contains the "worst thing in the world." He also
acknowledges that for every individual, this "worst thing" can vary quite widely
because our deepest fears are personal and subjective.

The fact that the
party knows the deepest fear of every individual in Oceania is indicative of the extent of its
control. It demonstrates the success of its surveillance methods, like the telescreen, and how
this information is manipulated by the party for the purposes of total control and
exploitation.

Thus, by using intelligence to create a complete picture of an
individual, the party can turn Room 101 into that individual's idea of hell. This is what makes
Room 101 so unique and also so successful.