Epistemology talks about the possibility of and criteria for knowledge.
Roleplayers are people who spend a significant amount of their time creating fictional characters and interacting with fictional worlds, often via the fictional characters. In my personal opinion, it would be foolish to not adopt a theory of knowledge that allows sentences such as “The lowly orc stabs Drizzt, who drops to his death in the gorge.” to be meaningful, and further makes most sense from the practical point of view.
The theory of truth often named correspondence says that a proposition (claim that something is true, essentially) is true if and only if it corresponds to the real world. This is, of course, totally useless to an average roleplayer. At least I hope the world isn’t really swarming with vampires who wield katanae, mages searching for Atlantis, werebeasts eating folk, etc.
Coherency is another way of defining truth; it claims that something is true as long as it is coherent with our other beliefs. I find this to be more usefu, as a roleplayer and especially GM. After I know something about a given setting, further additions can be evalued based on the coherency of them: If they conflict with existing facts, accepting them is kinda risky. Not always wrong, but risky nonetheless. Direct contradictions should be avoided almost always, but somewhat dissonant material can be useful.
Especially if Universalis or some other less extreme game where everyone gets to narrate is the order of the day, coherence is a pretty useful concept, also for checking the contributions of others. See also no myth as a related way of gaming.
There is at least one other theory of truth that springs to mind. I don’t remember what it is called, but the content is that truth is whatever works best. This, too, is a useful point of view for roleplayers. Usually, whatever works best, at least in the long run, is also coherent with the pre-established material.