What does"supreme law of the land" mean? What does the term supremacy mean?

Supreme law of the land: Since the Constitution is the supreme law of the land, those laws cannot be valid and must be overturned. So, the Constitution of the United States is the supreme law of the land, meaning that no law can be legal if it contradicts what the Constitution says.

Supremacy: the state or condition of being superior to all others in authority, power, or status.