Creatinine is pretty much all filtered, not reabsorbed - hence why it's used as a marker of filtration. The better your kidneys filter out things like creatinine i.e. your glomerular filtration rate, the less creatinine you should have in your blood. And conversely if your kidneys aren't working properly, they're not going to be filtering out things, including creatinine, so you'll have a high level and it will suggest your kidney function isn't so hot.
Creatinine relates to the on-going process of muscle turnover so greater muscle mass will give you naturally higher levels and vice versa, which can skew things a bit. Also ethnicity and some other factors. Very very tiny amounts of creatinine are reabsorbed but it's essentially negligible, although there is a biological test you can do using molecules which are even more exclusively excreted than creatinine. But even with all this in mind, for a given person creatinine is good to show relative changes in filtration and you can take them into account when considering an individual's creatinine, so basically it's a good surrogate marker of filtration for all these reasons.
That is my understanding of it, anyway.