The federal government is not supposed to be able to do whatever it wants. It is limited to a specific list, written down in the Constitution. The reach of that list is the oldest argument in American politics.
Enumerated powers are the specific authorities the Constitution expressly grants to the federal government, chiefly in Article I, Section 8: the power to tax, coin money, declare war, raise armies, regulate interstate commerce, and a handful of others. To enumerate means to count off one by one.
The design is one of limited government. The federal government is supposed to have only the powers listed, plus those reasonably implied by them. Everything else, the Tenth Amendment says, is reserved to the states or the people.
This was a core promise of the founding. To win over those who feared a too-powerful central government, the Federalists argued the new government could do only what the Constitution named, and no more.
In practice the list has proven stretchy. Through the necessary and proper clause and an expansive reading of the commerce power, the enumerated powers have been interpreted to reach far into modern life, which is exactly what strict-limit advocates have fought against for two centuries.
