The field of epistemology grapples with a fundamental question: how do we know what we know? And what does it mean to know something?
Humans are ambulatory machines that are really good at two things: secreting various fluids, and recognizing and interpreting patterns. Which is to say, we observe the world around us, we derive information from those observations, we develop working theories about how stuff works, and then we use that information to test the structure and consistency of the world.
This sounds really obvious, but it turns out to be anything but. At the core of mainline epistemological thought is the idea of the Justified True Belief; in other words, one believes something to be true, and if there is justification for that belief, it becomes knowledge.