In television terminology, a bible is a guidebook issued to the writers of a TV series, containing a detailed outline of the show's premise and setting, character profiles, and assorted rules and guidelines for the show. It can also apply to comics, novel, or any other type of media.
What, you want more information? Aren't you the greedy one!
But, anyways... the production bible contains the general backstory of the continuity in question and information on key locations, factions, and characters. Production bibles may also contain the "secrets" of a particular series, although sometimes these are never officially revealed.