When Winston and O'Brien are talking about Room 101 in Book 3, Chapter 5, O'Brien tells Winston that the worst thing in the world is in that room. He then goes on to tell Winston what that means in Winston's specific case. As he says to Winston:
'In your case, said O'Brien, 'the worst thing in the world happens to be rats.'
We have seen before that Winston has this horrible fear of rats. Having seen this, we are not surprised that this is what is in Room 101. But it is still pretty horrifying because of what O'Brien says will happen to Winston. Basically, they're going to put a rat in a cage, strap the cage to Winston's face, and let the rat go...
'You understand the construction of this cage. The mask will fit over your head, leaving no exit. When I press this other lever, the door of the cage will slide up. These starving brutes will shoot out of it like bullets. Have you ever seen a rat leap through the air? They will leap on to your face and bore straight into it. Sometimes they attack the eyes first. Sometimes they burrow through the cheeks and devour the tongue.'