A more thorough explanation:

The clear and present danger test is a standard used to determine whether a particular speech is protected by the First Amendment or not. It originated in the case of Schenck v. the United States and states that speech may not be restrained or punished unless it creates a clear and present danger of bringing about a substantial evil.

The clear and present danger test has two conditions: the speech must pose a threat of a substantive evil, and the threat must be real and imminent. The court must identify and quantify both the nature of the threatened evil and the imminence of the perceived danger.

The clear and present danger test has been applied in cases involving criminal prosecutions for opposition to war, statutes penalizing the advocacy of the overthrow of the government by force or violence, attacks on courts or judges, picketing, regulation of prison inmates' access to newspapers, periodicals, and so forth, incitement to commit crimes, and breach of the peace or disorderly conduct.

However, the rule has been held not applicable to cases involving antitrust laws, libel cases, statutes regulating the conduct of labor union affairs, statutes governing the use of school property for non-school purposes, and demonstrations in an inappropriate place, such as before a courthouse.

In Brandenburg v. Ohio, the defendant, a leader of a Ku Klux Klan, had arranged for a television station to cover his speech at a Klan rally. Ohio’s court ruled that the statement falls into the scope of clear and present danger.

In Hess v. Indiana, an anti-war demonstrator had been arrested for stating, “We'll take the fucking street later.” A majority of the Court reversed his conviction. The United States Supreme Court ruled that the statement is not a “clear and present danger” because the statement does not impose an imminent danger to society.

These examples illustrate how the clear and present danger test is used to determine whether speech is protected by the First Amendment or not. The Brandenburg case shows that speech that poses a clear and present danger can be restricted, while the Hess case shows that speech that does not pose an imminent danger cannot be restricted.