whatsa octave?

It's a musical range. Starting on a keyboard at the C key & moving up (do-re-mi, etc.) to the next C key is considered one full octave. I'm guesing whatever you're looking at is telling you how many octaves something is. It probably just means the range from low bass notes up to higher notes. A standard piano has 4 octaves I think.

I'm pretty sure I remember from my Stereo Review reading days, that in reference to audio equipment an octave is simply a doubling of the frequency. For example from 20hz to 40hz is one octave higher and so on.

At any given frequency..one octave above is 2 times as much, and one octave below is half as much. Forget about those technical stuff ,unless you want to build some crossover or speakers yourself. Octave use in speaker to measure the bass response drop off of a particular driver. It's a big headache, don't bother with it...