Glomerular filtration rate is not the same as a percentage function but it could be seen as somewhat similar. GFR is an estimate of the amount of mililiters of blood your kidneys filter every minute. If GFR = 20, then it would mean the kidneys filter about 20 mL of blood every minute. I'd say an average person has a GFR of about 100 mL / min so essentially you could almost look at kidney function of 20% = GFR of about 20 mL / min.
Hope that makes some sense.