The important legal principle that was set forth in this case was the principle of judicial review. This is the idea that the judicial branch (of which the Supreme Court is the highest level) has the right and the duty to decide when the actions of one of the other branches of government are unconstitutional. If the Court makes such a decision, the action of Congress (or the President) is nullified.
Today, we take this idea for granted. There have been many important Supreme Court cases (such as Brown v. Board of Educationand Roe v. Wade) in which the Supreme Court has struck down the actions of other parts of the government. But the Constitution does not explicitly give the judicial branch this power. In Marbury,Marshall simply claims the power of judicial review. He was successful in doing so and judicial review is now one of the fundamental ideas of our political and legal system.