"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
In plain English: "Anything we didn't mention in the Constitution is the domain of the states."
It's important to point this out explicitly, so that the federal government does not make assumptions of power if/when it encounters an issue not specifically enumerated in the Constitution.
Text:
"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
In plain English: "Anything we didn't mention in the Constitution is the domain of the states."
It's important to point this out explicitly, so that the federal government does not make assumptions of power if/when it encounters an issue not specifically enumerated in the Constitution.