A "claim" is something like--the producers let loose a pack of wolves in the studio and now I am disfigured, or they broadcast on TV that I am a cannibal and eat live children on Thursday nights, and my reputation is ruined. A "claim" is something that you can (normatively) sue ABC about. Bill Gates offering you $8 billion dollars is not a tort committed by ABC. "If I had known X, I wouldn't have done the deal" is not a claim. (Deliberately lying might be a claim, but that's a topic for another day...)
It's a convention in contract law (see meeting of the minds, unconscionability) that you have to have some idea of what you are signing for a contract to be valid, and there's a lot of arguing about what that means state-by-state, etc. California has specifically (attempted to) codify in Section 1542 that if a claim is "unknown" to you it doesn't meet that test. So if you are signing a waiver of liability of injury thinking about things like falling off a ladder, but not considering that the producers deliberately attack you with a pack of wolves, the latter might be an "unknown" claim that you didn't consider when signing the release. Under Section 1542, you could argue that even though you have signed a waiver, the waiver only covered things you would have reasonably considered, which do not include being deliberately attacked by wolves.
What this section does (IANAL) is say "Even though contract law doesn't let you waive a right you don't understand, and even though California has specifically exempted you from being responsible in this instance, fuck that, you're responsible." Which, I don't know if it is an enforceable clause or not, but it's certainly not moral, and it has nothing to do with "got a better deal, now I want out."