Marbury v. Madison is the US Supreme Court case in which laid the foundation for judicial review and confirmed the Supreme Court's ability to determine if any laws are unconstitutional, thereby further defining the boundaries between the executive and judicial branches of the US Government. John Marshall was the Chief Justice of the Supreme Court during the Marbury v. Madison case.
In his Opinion, John Marshall wrote:
"It is also not entirely unworthy of observation that, in declaring what shall be the supreme law of the land, the Constitution itself is first mentioned, and not the laws of the United States generally, but those only which shall be made in pursuance of the Constitution, have that rank."
In other words, his opinion is that Constitutional law supersedes that of the federal government and if the Judiciary Branch (with the Supreme Court) find a federal law to be in conflict with the Constitution, the law of the Constitution will be upheld first and foremost.