What is epistemology? Why is it important to critical thinking?

Epistemology can be defined as the branch of philosophy which focuses on the theory of knowledge. It is the study of the nature of knowledge, and analyses its relation concepts like truth, justifications and belief. It deals with the rationality of belief (logic or rationale behind a belief).

Yes, it is important to critical thinking as critical thinking is based on beliefs and actions that have reasons, logic and rationale behind them. But these logic and reasons are philosophically problematic as how one evaluate 'reason, 'logic', etc. These are abstract and difficult but question like these are a central part to epistemology.