What did Marshall mean by it is duty of the judicial department to say what the law is?

When Chief Justice John Marshall stated that it is the duty of the judicial department to say what the law is, he emphasized the role of the judiciary in interpreting laws and determining their constitutionality. This principle, established in the landmark case Marbury v. Madison (1803), underscored the judiciary's responsibility to ensure that the law is applied fairly and consistently, serving as a check on the legislative and executive branches. It affirmed the judiciary's power to review and invalidate actions that conflict with the Constitution, thereby reinforcing the rule of law.