When the Supreme Court says a law is unconstitutional it means?

Exercising its check on the power of the legislations.

Explanation:

The supreme court belonged to the judicial branch of the government. As a form of check and balances, the judicial branch has the power to check the legislative branch by conducting a judicial review.

In a judicial review, the supreme court will evaluate the laws proposed by the congress and see if any part of the law proposal violates the United States constitution. If there's a part that violate it, the supreme court can deemed the proposed law as "unconstitutional" and the law cannot be passed by the Congress.

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