Is there a place that explains all the sound terms, like mid, soundstage, etc?

Thanks a lot! 

I don't understand mid from that link coz it states the frequency only

mids are mostly vocals (maybe male vocals?  )
if you want to understand where those frequency derscribed lies, open up a player with frequency equalizers and you will notice they are the middle 4 bars of an eq, adjusting it increases the vocals, piano sounds or something orchestral that is not high pitched

I don't understand frequency for now. From what I've learnt, low frequency produces bass, and high frequency produces treble. Am I right?

may not entirely that low freq = bass, deep male vocals are also in the low freq spectrum, and same goes to female vocal