A writers' bible is a reference guide given to writers for a series, describing the worldbuilding, characters, emphases, and limitations that the producers desire to see expressed in scripts. At the link, you'll see the writers' bible for Batman: The Animated Series -- the excellent 90s cartoon that formed the first pillar of the DC Animated Universe. If you're a fan of the series, it makes for fascinating reading. Here's a sample.
Our version of Batman will rely less on gadgets and more on his own deductive powers and fighting skills. Staying true to The Batman's original conception as "the world's greatest detective," we want to stress his mental abilities, whether he be assembling clues in a case or using his vast scientific knowledge in the Bat cave's laboratory. He is a crime-fighter who is skilled in many languages and is also a consummate master of disguise.
Link via io9