What are the political prisoners afraid of in 1984?

There are at least a couple different ways to answer this
question.  (I assume you are asking about what we are told in Part 3, Chapter
1.)

The simplest answer is to say that the political
prisoners are afraid of Room 101.  It is clear that something completely terrible
happens in that room.  We see many examples in this chapter of how afraid the political
prisoners are of that room.

At a deeper level, though, this
question could be asking why the political prisoners are afraid while the common
criminals are not.  I would argue that this is because the political prisoners are more
of a threat to the Party and therefore they are treated much more harshly.  The
political prisoners were members of the Party (and are also referred to as "Party
prisoners").  They have committed the more serious (to the Party) offense of
thoughtcrime.  The Party sees the politicals as "adults" who have openly rebelled
against the Party.  By contrast, the common criminals are more like children who simply
don't know any better.  Because of this, the political prisoners are a bigger threat and
must be punished more harshly.  This is why the political prisoners are
afraid.