Abrams announced the project with a mysterious tweet that lead fans to the hidden YouTube trailer, above.
What is Castle Rock? Actually, it's a where: the Maine city is where King set some of his stories, including "The Body" (which was developed for the screen into Stand By Me), The Dead Zone, Needful Things, Cujo, and The Dark Half, among others. It's made an appearance, or made mention of, in many of King's other work.
The series looks to be an anthology of those stories around Castle Rock, so if you're into his universe you might see some familiar characters pop up (like the narrator from The Shawshank Redepmtpion, who was from Castle Rock), or the corners of old stories making their way into this series.
It all ties together in one big web — but, for what it's worth, the web you see in the series trailer is in fact, a series of connected blood vessels. So creepy.