The Supreme Court is the highest court of the United States. The Supreme Court decides if a law goes against the Constitution. All other courts must follow the decisions of the Supreme Court.
One example is the case of Brown v. Board of Education of Topeka in 1954. The case was about racial segregation in schools. Segregation was a system that separated people based on race and ethnic group. The Supreme Court decided that segregation in schools was against the Constitution. All states had to follow the Supreme Court decision. Segregation in school became illegal in the United States.