A more thorough explanation:

The advocacy of illegal action refers to speech that promotes or encourages illegal behavior. This type of speech is not protected by the First Amendment, which guarantees freedom of speech in the United States.

For example, if someone gives a speech urging people to commit acts of violence or to break the law, they could be prosecuted for advocating illegal action. However, the Supreme Court has established a test to determine when speech crosses the line from protected speech to illegal advocacy.

The Brandenburg Test, established in the case of Brandenburg v. Ohio, requires that speech must be intended to incite or produce imminent lawless action, and likely to incite such action, in order to be punishable by law. This means that simply expressing an opinion or advocating for a cause, even if it is controversial or unpopular, is protected by the First Amendment.

For example, if someone gives a speech advocating for the legalization of marijuana, this would be protected speech because it does not meet the criteria of the Brandenburg Test. However, if someone gives a speech urging people to start a riot or to commit acts of terrorism, this would not be protected speech because it meets the criteria of the Brandenburg Test.

In summary, while the First Amendment protects freedom of speech, the advocacy of illegal action is not protected. The Brandenburg Test is used to determine when speech crosses the line from protected speech to illegal advocacy.