The Court and Constitutional Interpretation
"The complex role of the Supreme Court in this system derives from its authority to invalidate legislation or executive actions which, in the Court's considered judgment, conflict with the Constitution. This power of "judicial review" has given the Court a crucial responsibility in assuring individual rights, as well as in maintaining a "living Constitution" whose broad provisions are continually applied to complicated new situations."
Simple terms....
Role of the Supreme Court | Scholastic.com
"It can tell a President that his actions are not allowed by the Constitution. It can tell Congress that a law it passed violated the U.S. Constitution and is, therefore, no longer a law. It can also tell the government of a state that one of its laws breaks a rule in the Constitution."