For a really non-precise but still general-gist-of explanation, its the information in the game that isn't necessarily available to the players in-game but is nonetheless present at all times. Many add-ons make that information available to players, as long as ZOS allows that info to be used. So the DPS counters like FTC and Combat Metrics use it, and so does Miats. Miats' add-on actually made ZOS reevaluate what info was available (like the stealth alerts and they keep going "well, we're considering it" for some time.