A more thorough explanation:

The Cooley Doctrine is a principle in constitutional law that states that Congress has the exclusive power to regulate both national and local aspects of national commercial matters under the Commerce Clause. However, states may regulate those aspects of interstate commerce that are so local in character that they require diverse treatment.

For example, if a state wants to regulate the transportation of goods within its borders, it may do so as long as it does not interfere with Congress's regulation of interstate commerce. However, if a state wants to regulate the transportation of goods across state lines, it must defer to Congress's authority under the Commerce Clause.

It is important to note that the Supreme Court has abandoned the Cooley Doctrine in favor of a balancing test for Commerce Clause cases. This means that the Court now weighs the state's interest in regulating a particular aspect of interstate commerce against Congress's interest in regulating that same aspect.