A more thorough explanation:

The Dormant Commerce Clause is a constitutional principle that prevents states from regulating interstate commercial activity, even when Congress has not acted to regulate that activity under its Commerce Clause power. The Commerce Clause, found in Article I, Section 8, Clause 3 of the U.S. Constitution, gives Congress the exclusive power to regulate commerce among the states, with foreign nations, and with Indian tribes.

For example, if a state were to pass a law that restricts the transportation of goods across state lines, that law would likely be struck down as a violation of the Dormant Commerce Clause. This is because the regulation of interstate commerce is within the exclusive purview of Congress, and states cannot interfere with that power.