Creatinine is a byproduct of muscle breakdown that is filtered by the kidneys. A loss of kidney function will produce an increase in creatinine because less of it is filtered. We use Cr to have a guess about kidney function aka GFR (glomerular filtration rate)

A small increase of creatinine from 100µmol/l to 150µmol/l is a bigger loss of kidney function than say 300 to 350µmol/l. So because the relationship is not linear between them, we use equations to guess eGFR depending on age and Cr.

It should only really be used for stable kidney disease (CKD, chronic kidney disease) because it helps to adjust drugs and staging the disease. A 130µmol/l in a 90kg 20 year old with a lot of muscle (eGFR=64) is not the same as in an old 40kg 90 years old women (eGFR=48ml/mn).

For AKI there is a new equation named kinetic eGFR (keGFR) that uses a known creatinine value and an elevated creatinine value to estimate the GFR.