Creatinine and GFR Levels

I have many health problems, but I am very curious about a recent cbc test I had performed which showed a couple figures that bothered me a little. I have always had trouble with my bladder being overactive. When I was younger and more sexually active I had a lot of UTI's and even was hospitalized once for painful bladder spasms (the doctors thought at first I had a severe uterine infection because my stomach was so bloated).

I drink a lot of fluids all day- all sorts of fluids caffeinated and non caffeinated- depending on what part of the day it is. I have tried cutting out the caffeine, but it doesn't really matter. As you can imagine I use the bathroom a lot- even at night. :)

Anyhow, my creatinine level is .06 mg/dl, which is low, but my doctor wasn't worried since I drink so much. I was wondering though about my GFR level though because I don't know what it means. It's 115. I am a white female (if that makes a difference). Could someone explain to me what a GFR level is and what my level means? Also, why am I drinking so much all of the time? Thanks.