My Dr said one Kidney is doing 70% and one is doing 30% what does this mean?

I had routine blood work and urinalysis where proteins where found in the urine. So the Dr did a kidney ultrasound and said 1 kidney was enlarged so he did a renal scan. I went back to se him yesterday and he said 1 of my kidneys doesn't work very well and is only doing 30% of the work and the other 70%. What does this mean? He was very elusive and told me that this is something for him to worry about . Than he sent me for another creatin clearance test. ( which probably is ok as I haven't hear ed back) Does all this mean that 1 day my kidneys will fail? Or are failing? Should I be eating a special diet? Should I limit my protein?