If you are asking about the United States, the answer is that the supreme law of the land is the Constitution of the United States. This is laid out in Article VI of the Constitution itself. Part of that article says that
This Constitution, and the Laws of the United States which shall be made in Pursuance thereof … shall be the supreme Law of the Land…
What this means is that no law made by any state, or even by the Congress of the United States, can contradict the Constitution.
This is why the Supreme Court of the United States is able to overturn laws by declaring them to be unconstitutional. Those laws are, the Court says, contrary to the Constitution. 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.