Freedom of speech under the First Amendment. It means the government can't arrest you for speaking your mind (with some exceptions regarding criminal plotting and incitements to riots, or, during the red scare, being a commie). That's it. No arrest or fining. No formal punishment from the government.

Private entities can do whatever they want; facebook can do what it wants, reddit can do what it wants, and anyone can criticize or ostracize anyone else for what they say. It's freedom of speech, not freedom from consequences. The government won't jail you but society can still condemn you.