What is the Supreme Law of the Land in the US and how does it affect what the States can do and laws States might pass?

The Supremacy Clause is a clause in the United States Constitution. The clause establishes the Constitution federal statutes and U.S. treaties.

Some laws are different in different states. For Example, a law in California could not be a law in Maine.

However, ALL State Laws must be Constitutional, or can be overturned through the US Supreme Court which through its reviews and rulings safeguard the US Constitution's meaning and intent.