What is this plugin?How do I start it?Will I get good Stuff?what's it's objective?Does it Add good things to the game?

I'm on the ps3...

KotN is like the Dawnguard of Oblivion: Has a small amount of content but nothing like a new landmass like SI. How you start it is you go to Anvil and look for a Prophet screaming about the end of the world (He is near the Chapel).

The rest is up to you to find out

Go to Anvil. Ask around about Anvil and the Prophet.
Yes, there is good stuff.
Walk the path of the virtuous.