A more thorough explanation:

Definition: The Riot Act was a law passed in England in 1714 that made it a serious crime for 12 or more people to continue rioting for an hour after a magistrate has ordered them to disperse. Although this law was not adopted in the United States, the phrase "reading the Riot Act" has become a part of the English language and means to reprimand someone severely.

Example: When the police arrived at the scene of the protest, they read the Riot Act to the crowd, warning them to disperse or face arrest.

Explanation: In this example, the police are using the phrase "reading the Riot Act" to warn the protesters that they must leave or face legal consequences. The phrase has become a way to describe any situation where someone is being sternly reprimanded or warned.