CNBU stand for "Can Not Be Unseen", the purpose being to distinguish between that which is appealing and NSFW and that which is disturbing/grotesque/just plain wrong.
"How does this affect me?" you ask. In short:
Good NSFW remains NSFW; bad NSFW turns into CNBU.
Goatse is now CNBU... unless you're into that sort of thing.
Can Not Be Unseen
Good NSFW == NSFW; Bad NSFW == CNBU