"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."
Analysis of the 10th Amendment
Basically, what the 10th Amendment means is that powers not temporarily
granted to the federal government by the Constitution and/or the Bill
of Rights, are still the property of individual States within the union,
or, in the case of those powers that were prohibited by the Constitution
to the States, those powers were still the property of the people at
large.
"Powers" deals with
things that you can do due to the temporary assignment of your Rights
to someone else.
Police powers are one of the more visible powers that everyone has
naturally and that have been temporarily assigned to the federal government,
State governments, and local governments.
Why is this? Because -you- have the right to protect your own property.
If you had to do it yourself, you'd have to stand guard 24 hours a
day on your property. So, in order to make a living, you wisely decide
to temporarily assign the right to protect your property to the local,
State, and national governments. They create police departments to
do this work. Note also that this does not remove those rights from
you. You still have the right to protect your own property, despite
your decision to temporarily assign those rights to the government.
Let's try a more modern approach - and once again remember that this
is one man's opinion of what the Bill of Rights means.
A Modern 10th Amendment
Rights not given temporarily to the federal government by the Constitution
are held by the States individually, and those rights specifically
not temporarily granted to the States by the Constitution are held
by the people.
What does this mean? It means (among other things) that the programs
that the federal government currently -mandates- that States adopt
(or federal funds will be withheld) are unconstitutional.