How serious is renal failure?

It depends. Usually it depends on the reason , the severity and person health. While renal failure due to dehydration and obstruction ( stones, prostate in a man)is reversible most of the the time , renal failure due to intrinsic kidney problems is sometimes hard to treat.
Depends on severity. In short, renal failure is not good. A general measure of the degree of renal failure is estimated glomerular filtration rate, usually calculated from serum creatinine levels. An egfr value above 90 is normal, above 60 is not too bad, below 60 has serious implications for health and life span. If you have any concern about renal failure, see your doctor.