What did Marshall mean by the phrase "It is emphatically the province and duty of the judicial department to say what the law is"?

In the Marbury v. Madison Supreme Court Case, the concept of judicial review was firmly established.  Judicial review is the idea that the courts can rule a law that has been passed by a legislative body as invalid.  This ruling is made on the basis of Constitutional law.  In other words, if a law is passed that violates the Constitutional rights of citizens, it can be overruled by the courts.  The Supreme Court has the authority to ultimately make this decision.  

The concept of judicial review is often criticized by political analysts as "ruling from the bench" or in giving the judicial branch too much authority.  The statement of Chief Justice John Marshall that is referenced in the statement above seems to confirm these fears.  What Marshall meant is that when two laws seem to contradict one another, it is the duty and responsibility of the courts to decide which law should be valid.  This is especially true if the law in question is in contradiction to the United States Constitution.