"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." Amendment 10 of the Bill Of Rights
This is what it says. The 10th Amendment is a very clear and simple statement - there is no ambiguity. Until passage of the Bill Of Rights, the U S Constitution stated:
"The Congress shall have Power To lay and collect Taxes, Duties, Imposts and Excises, to pay the Debts and provide for the common Defence and general Welfare of the United States;..." Article 1, Section 8, Clause 1
The 10th Amendment amended all statements in the Constitution regarding federal authority and jurisdiction.
To "provide for the... general welfare of the United States" gives Congress power to pass any legislation they want, as long as they can argue that it is good for America. The 10th Amendment limited Congress to only those powers specified in the U S Constitution, which are enumerated in Article 1, Section 8. These are the only powers of the federal government, all other legislation is reserved for the states or the people.
The 10th Amendment is written law. Until it is amended, this is what it says. It doesn't really mean the federal government can overwrite the U S Constitution because 51% of the people thinks it's important or necessary. We have a process, written into law in the U S Constitution, that provides for changes - Article 5 of the U S Constitution.
"The Constitution is a written instrument. As such, its meaning does not alter. That which it meant when it was adopted, it means now. - South Carolina v. United States, 199 U.S. 437, 448 (1905)."
"The Constitution was written to be understood by the voters; its words and phrases were used in their normal and ordinary, as distinguished from technical meaning; where the intention is clear, there is no room for construction, and no excuse for interpolation or addition." - Cooley's Constitutional Limitations, 2nd ed., p. 61, 70.