Answer:
Well, in the Prisoner of Azkaban, we are introduced to a Boggart. No one knows what they really look like, they are shape shifters and take the form of the person's greatest fear, that's what makes them so scary.
In this movie we see Harry go up and the Boggart turns into a Dementor.
Later, Professor Lupin (Remus Lupin- the Defense Against the Dark Arts Instructor) clarifies that it shows that Harry is afraid of fear itself.
Step-by-step explanation:
Fear Itself, (and dementors)