Fear

   In my book this scene happens on page 177. It when Harry and Lupin are discussing the boggart and Harry confesses that his biggest fear is of dementors. Lupin's response is as follows, "That suggests that what you fear most of all is - fear. Very wise, Harry."
   This comment shows Lupin's wisdom of the "ancient magic." Ancient magic is not so much magic but power, and it's power not limited to Wizards but available to muggles as well. The greatest of these powers is love but fear is among the greats as well.
   When we fear something we are giving it a certain measure of control over ourselves and our actions. The magnitude of this control depends on the magnitude of our fear. In order to avoid it we may go somewhere we may not have otherwise have gone or not go somewhere where we would have gone. Our actions reflect our fears because we often change our behavior in order to avoid our fears. For example, if you see something rustling around in the bushes you may be curious as to what it is but you fear of the unknown and getting hurt causes you to walk away rather than investigate.
   Of course, fear can work in much more impactful ways as well. Say you lived somewhere where Christians are persecuted for their faith. Perhaps you would not follow Jesus because you fear personal harm or separation from friends and family. In this situation you would be allowing your fear to control your eternal destiny.
   If what we fear most is fear itself then we have recognized that we cannot focus our attention on overcoming our fear of spiders or pain or another individual thing and expect to become fearless or courageous. In fact, if we are fearless, we have not yet recognized/acknowledged danger and we are, in that way, quite unwise. You cannot boast of being courageous and fearless because you need fear to have courage.
   Courage is not the absence of fear, nor is fear the absence of courage. Courage is overcoming fear. Courage is being willing to ignore your fear for the good of something else. Lilly Potter, for example, was courageous when she didn't allow her fear to prevent her from protecting her son although she had the option to do so (and we know that she definitely feared death/Voldemort or she wouldn't have put the fidelius charm on her). Although she died, her courage was rewarded because Harry survived.
   So, in conclusion, If you fear fear above all else, what you are most afraid of is allowing your feelings to stop you from doing what is good and important. This fear is very wise because it shows we have recognized that as long as there are things we love, there will be things we fear and we have agreed to not allow our personal fears to prevent us from fulfilling our purpose for the greater good.

(p.s. I realize that "for the greater good" may have a negative connotation for any major Harry Potter fans but please realize that the saying itself is actually a good one when used in a context where you are actually working for the greater good and generally requires you personally to make sacrifices rather than sacrificing others.)