By the process of Judicial Review, the Supreme Court - as the custodian of the Constitution - can analyze a law to find out if it goes against the principles enshrined in the Constitution.
If they find that the law in question has gone against the Constitution, they can declare that law to be null and void. For instance, the Supreme Court found in Brown v. Board of Education that segregated schools were unconstitutional and this led to schools being desegregated nationwide.
