The judicial branch of the government interprets laws. When cases are brought to the Supreme Court (or any other lower civil court in the United States), it is the Court's (judiciary's) job to interpret the relevant law in the case. The Court might decide that the law is being unfairly executed or that the law was unconstitutional to begin with. Whatever the Court decides, this becomes the precedent for how the law must be interpreted. Since the Supreme Court is the highest court in the land, it can determine how laws must be interpreted for the entire country.