For example, the books written by Martin are the "Bible" for Game of Thrones, the books written by Tolkein are Jackson's Bible for the LotR movies. It's just a metaphor.
Actually, there IS a series bible for Game of Thrones (and it's not the books), but LOTR would not have had a series bible, but a continuity bible -- different animal, altogether.
A series bible is the document writers of a series use to familiarize themselves with the world of the series. It contains a "mission statement", format information, plot synopses of the episodes, character bios and assorted essays or glossaries. For example, the 4th season bible for the new Battlestar: Galactica contained essays on how the show's SF technology was supposed to work, techno-babble, Cylon culture, etc., as well as the standard synopses/bios.
A Conan series bible might have The Hyborian Age essay, bibliography, essays that would help writers capture the feel of REH, and synopses and character bios. The LOOK of a show is not contained in a bible, as far as I know. That's the purview of the production designer, set designers, costumers, etc.
Hope this helps!
Scott