The supreme law of the land refers to the U.S. Constitution and any federal laws and treaties based upon it. In short, it means that constitutional or federal law is upheld over state law.
The "supreme law of the land" is noted in the Supremacy Clause of the Constitution, which is found in Article VI, Clause 2. It means that federal law overrides individual state's laws if a conflict in statute occurs. It also requires state judges to uphold federal law over state law thereby making it the supreme law of the land. The Supreme Court interprets and upholds constitutional law.