When something is unconstitutional, it violates the rules laid out in a constitution, a foundational document that establishes how a government works and protects citizens' rights. Many countries have constitutions, though this term is most commonly used in the United States.

The U.S. Constitution is like the ultimate rulebook for American law: any law, government action, or policy that conflicts with it is unconstitutional and therefore invalid. For example, if Congress passed a law saying the government could arrest people for criticizing the president, that would be unconstitutional because the First Amendment protects freedom of speech.

The Supreme Court has the power to declare laws unconstitutional through a process called judicial review. When the Court rules something unconstitutional, that law or action must be stopped or changed. Think of it like a referee in a game: even if players agree on a rule, the referee can call it out if it breaks the official rules.

Understanding what's constitutional helps citizens know their rights and hold their government accountable to the rules everyone agreed to follow.