What is the significance of the supremacy clause?

A supremancy clause allows the National Government to out do smaller levels of Government, in doing so can prevent unfair policies and a lack of justice in that form of Government that's all wrong  Simply StatedThe Supremacy Clause makes invalid any law passed by a State or local government that is in conflict with a federal law. In other words, federal law is "supreme."
Though that concept seems simple, it's application is not. So don't start throwing around the "supremacy clause" unless you understand how the courts apply it. The importance of the Supremacy clause is that it establishes that the Constitution and Federal Law are the absolute law of last resort in the United States. By that, it means that the Constitution or Federal Law will win in any conflict between it and any state or local law. Thus, all states are bound to the limits placed on them by the Constitution and federal law.
In practice, here's what it means: no state or locality can pass a law which prohibits something expressly allowed by the Constitution or Federal law, nor may a state or locality allow something expressly forbidden by the Constitution or Federal law.