A more thorough explanation:

The Supremacy Clause is a part of the U.S. Constitution that says federal laws and the federal constitution are more important than state laws and state constitutions. This means that if there is a conflict between a federal law and a state law, the federal law wins.

For example, let's say a state passes a law that says it's okay to sell marijuana for recreational use. But federal law says that selling marijuana is illegal. In this case, the federal law would be more important than the state law because of the Supremacy Clause.

The Supremacy Clause also says that states can't do things that only the federal government is supposed to do. For example, only the federal government can make treaties with other countries. So a state can't make a treaty with another country because that's something only the federal government can do.