It is reference to Article II of Confederation:
“Each state retains its sovereignty, freedom, and independence, and every Power, Jurisdiction, and right, which is not by this confederation expressly delegated to the United States, in Congress assembled.”
The anti-federalists demanded insertion of this Article as an amendment to the Constitution.
I much prefer the 10th amendment to Article II. Art. II enshrined the supremecy of the state over the people. The 10th Amendment holds both the Federal and State governments to their constitutionally enumerated powers and reserved all other powers to the people.