A more thorough explanation:

Darby v. United States is a legal case that dealt with the Fair Labor Standards Act of 1938 and federalism. The Fair Labor Standards Act set federal standards for employment conditions, such as minimum wage, maximum hours, and child labor. Congress used its powers under the Commerce Clause to prevent goods produced under employment conditions that do not meet federal standards from entering interstate commerce.

The United States government sued Darby Lumber Company in the District Court for the Southern District of Georgia, alleging that Darby did not meet the requirements of the Fair Labor Standards Act while using interstate commerce. The District Court dismissed the indictment in favor of Darby, holding that the Tenth Amendment prevented the Government from meddling in intrastate matters.

The Supreme Court unanimously reversed the District Court, holding that the Fair Labor Standards Act was constitutional because the Commerce Clause allowed the Government to regulate employment standards in the production of goods that touch interstate commerce.

For example, if a company in Georgia produces goods using sub-standard labor practices and sells those goods to other states, the Commerce Clause gives the Government the power to prevent that company from gaining an advantage in interstate commerce.

Darby v. United States expanded Congress' powers under the Commerce Clause with regard to economic legislation.