Midrange, simple as that.
You need mids to "cut live" because otherwise, those midrange frequencies are absorbed by all the people that make up the audience. So you boost the mids so you can hear more of the actual notes being made on your guitar, bass etc.
The EQ is basically a volume control for specific frequencies, or specific ranges of frequencies. It's like making particular strings or points on the fretboard sound louder, as well as those harmonics that are resonating. If you turn up certain midrange frequencies, you'll hear more of the harmonics. If you turn up other midrange frequencies, you hear more of the fundamental frequencies of the notes.
I think that's why its such a tricky balance, hence all the fuss with mid-scooping and mid-boosting and "cutting live" etc. If the harmonics sound louder, it sounds more distorted, but eventually at the expense of not hearing the original note (therefore becomes fuzzy and all mush). If you go the other way, it sounds too "clean" and maybe muddy, like there's no character or edge to the notes.
I made a chart once of all of the frequencies of each note for a 24 scale fretboard. But I think off the top of my head that the low E open string is 82 Hz.
