A more thorough explanation:

The Negative 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 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 import or export of goods across state lines, it would violate the Negative Commerce Clause because it would interfere with interstate commerce.

The Negative Commerce Clause is also known as the Dormant Commerce Clause.