#2
you can use it to fiddle with the frequenses like more high and low or more mid only a bit more precise, and use a search button next time
Wise Man Says: The guitar is obviously female, she's got hips, breasts... and a hole.
UG's Flamenco Club
#3
boosts or cuts frequencies (bass-mids-treb etc) changing the sound

no doubt someone should provide you with a more scientific answer soon enough
#6
In case you're wondering what is a good one to buy (you might not be, which is fair enough)

I HIGHLY recommend Behringers EQ pedal. I know they make a lot of garbage, but I got this pedal the other day and it is shocking to see what a quality pedal you can get for about £10

and its completely noise-free