The Mayo clinic (mayoclinic.com) is an excellent resource for pretty much any illness or tests. The explanations are very helpful here is what they say about creatinin:
A creatinine test reveals important information about your kidneys.
Creatinine is a chemical waste product that's produced when you use
your muscles or eat meat. Healthy kidneys filter creatinine and other
waste products from your blood. The filtered waste products leave your
body in your urine. If your kidneys aren't functioning properly, an
increased level of creatinine may accumulate in your blood. A serum
creatinine test measures the level of creatinine in your blood.
Creatinine can also be measured in your urine.
Because diabetes can damage the kidneys, regular creatinine tests
are often recommended for people who have type 1 or type 2 diabetes. If
you have kidney disease, creatinine tests can help your doctor monitor
your condition. Creatinine tests can also impact your treatment plan,
since several medications may not be safe to take if your kidneys
aren't working well.
I hope that is helpful.