what is fear?

[anagogy]

does anyone have a metaphysical understanding of what fear is?

As other posters have already said, fear is the visceral reflection, felt in your body as a kind of chemical cocktail, in response to your inner perception of separation.

Separation from what?  It is the separation of self from self.  All emotions arise in response to what you are focused upon.  When your focus, in this moment, is different from what your higher self has currently expanded to (what it is focused upon), you will feel negative discordant emotion.  If it is *really* off, you will feel dread or fear.  Just as physically aversive sensations like pain tell you when you burn yourself, the negative emotion is telling you "This is painfully HOT!", but in a spiritual complex sense.

Experienced occasionally, it is no big deal, and is a natural part of the process of giving birth to new desires as a result of catalyst/contrast, and the subsequent tuning one's thoughts to be a vibrational match to the new place that the higher self has expanded to as a result of your experience.  

Fear is a valuable indicator.  It is like the empty indicator on your fuel tank.  You can't just slap a happy sticker on your fuel gauge and keep driving.  It is useful to know when your tank is on empty, so you can fill it up again!