How do I write a high level functional description of a nerf blaster?

You simply have to describe in detail the mechanical way (the functions) that a nerf gun works. High level simply referes to the level/amount of detail required.

For example this is a (partial) high level functional description of a single action pistol trigger:

"The act of inserting the magazine and operating the slide mechanism to chamber the first round also cocks the hammer or striker into the ready-to-fire position). Once the first round is fired, the automatic movement (recoil) of the slide cocks the hammer or striker for each subsequent shot. The pistol, once cocked, can be fired by pulling the trigger once for each shot until the magazine is empty."