Self explanatory, what are impurities? Got a few elixirs with those as the penalty for their permanent stat boots but i'm reluctant to test them because i don't know what affect it has.

You will need 150 of them to rebirth later on. One of the big tropes in the Cultivation genre is taking an herbal bath to purge impurities from your body in order to make it stronger. You technically always have them until you do that, but I think this is a good way to combine the pill taking trope and the impurity purging trope together without bogging you down with also trying to find the ultra rare herbs you need for the bath.

Correct, Impurities are tied to the rebirth mechanism once you have perfected your foundations ( max Soul/Body rebirth)