They're making it into a movie. I'll wait for the movie.
If you wanna know about the Da Vinci code without the fiction fluff, rent the documentary on it.
Regardless of the Priori (which Da Vinci allegedly belonged to), Knights Templar or Gnostic Revelations, it's basically a line of thought that Jesus had a lover in Mary Magdalane and produced offspring which were the Morovian Kings, which super secret powerful/rich societies tried to protect this secret (for risk of persecution from the Vatican) while at the same time trying to bring these Jesus descendants to "rightful" throne of power in European society. The Da Vinci code is basically secret code based on fun math (like the "Mona Lisa", which is coded to signify that it's really a portrait of himself and not a woman). Some of his paintings (like "The Last Supper") show his extreme distaste of the Vatican and even Jesus for having a lover in Mary--also with coded clues.
There's also speculation that these secret societies like the Templar supposedly found the Holy Grail in Jersualem or ancient sacred documents attesting to the above or mystical powers of alchemy on how to create gold.
Dan Brown is obviously having a lot of fun with this material.