The Commerce Clause, of course, is in the Constitution of the United States of America. Most national law in America is enforceable inside the States because it is tied ("pegged") to the Commerce Clause. This is the full extent of the clause, from Section 8 of Article I:
According to Morris, in legal jargon "dormant commerce clause" does not just mean that Congress is not exercising its power to regulate commerce. The theory of the dormant commerce clause went through three phases. In the first phase, starting in 1829 with a Supreme Court decision, dormant did mean Congress had made no law about a particular commerce issue. The Court ruled that if Congress did not act, individuals States were free to act to regulate their own commerce as they saw fit.
"Congress Shall have power ... to regulate commerce with foreign Nations, and among the several States, and with the Indian tribes;"
In Phase II, running from 1851 to until 1876 (a period of time when corporations, monopolies and trusts came to dominate the American economy), the Supreme Court ruled that when Congress was silent, it was up to the Supreme Court to decide whether states could act. The Court wanted to help corporations reach national markets. It started interpreting free trade to mean not just that goods were not taxed when they moved across state borders, but that local laws and regulations that impeded any business operating from out of state were unconstitutional.
In Phase III, starting in 1876 and running up to the present, dormancy and the Constitution were turned on their head. The Supreme Court ruled that when Congress was silent, the states could make no law regulating trade (or a particular article in commerce). States could only regulate trade to the extent Congress specifically allowed to them.
With that turn the power granted to the federal government to regulate commerce "among" the states became the power to regulate all commerce, including commerce that stays in any particular state.
In the 1960's civil rights legislation needed to go beyond asserting the right for all adults to vote (until then Democratic Party denied non-white Americans the right to vote in the former states of the Confederacy). The Federal Government wanted to say that a black American could go anywhere a white American could go. If the owner of a particular restaurant in a particular state did not want to serve food to blacks, the federal government wanted to be able to prosecute the owner.
Rather than basing such legislation on the 14th Amendment, it was based on the Commerce Clause. The idea was that Congress can regulate all commerce, so it could regulate commerce at a particular restaurant, so it could say black Americans could eat, or work, at that or any other restaurant or establishment.
Racist segregationists asserted "states' rights," correctly pointing out that the commerce clause was not meant by the writers of the Constitution to allow the Federal Government to boss around the owner of a small store.
Civil rights activists and sympathizers defended the federal interference with local business people, and thus defended the dormant commerce clause. For decades anyone who mentioned "states' rights" was assumed to be, at least secretly, a racist.
Yet the right to abolish slavery in a state, before the Civil War, had been states' rights. The infamous Dred Scott Decision (brought to you by the Democratic Party) declared that northern states had no right to make slavery illegal inside their borders.
Powers are two edged swords. Most people are for states rights when that gets them what they want; against states rights when that would go against my desires. The Supreme Court is no different. There was a fear that the current conservative court might be generally sympathetic to states rights arguments. Instead they have favored states only when that helped their conservative agenda. With respect to the commerce clause, they have no interest in states rights.
See also states' rights at wikipedia