Defamation occurs if you make a false statement of fact about someone else that harms that person’s reputation. Such speech is not protected by the First Amendment and could result in criminal and civil liability. Defamation is limited in multiple respects though.
If you make a false statement of fact about a public official or a public figure, more First Amendment protection applies to ensure that people are not afraid to talk about public issues. According to New York Times v. Sullivan (1964), defamation against public officials or public figures also requires that the party making the statement used “actual malice,” meaning the false statement was made “with knowledge that it was false or with reckless disregard of whether it was false or not.”
Parodies and satire are protected by the First Amendment (and are not defamatory). Parodies and satire are meant to humorously poke fun at someone or something, not report believable facts.