Epistemology is the study of knowledge: what we know, how we know it, how we know we know it, and how to keep track of it without driving ourselves crazy.
You might be wondering if epistemology is just a hobby for people who know too much for their own good, but epistemology has a lot of uses. The study of knowledge leads to the study of learning, which leads to better methods of teaching. The study of knowledge helps us understand our cultural differences, which helps us all get along. Probably the coolest use of epistemology, though, is artificial intelligence: teaching computers how to learn.