Opinion privilege is a protected form of speech, of importance to US federal and state law.
The US First Amendment guarantees free speech, subject to certain limitations.
One of these limitations is defamation, in various forms, notably libel.
While federal precedent does not explicitly state that opinion is protected against prosecution under libel laws (indeed it explicitly states the contrary), the combined effect of several rulings is such as to effectively make such the case.
Opinion privilege has its roots in the common law fair comment doctrine.