In it, the court upheld "yellow dog" contracts that forbid workers from joining trade unions.
William Adair, an official with the Louisville and Nashville Railroad, had fired O. B. Coppage for belonging to a labor union. Adair's actions were in direct violation of the Erdman Act of 1898, which at the time prohibited railroads that engaged in interstate commerce from requiring that their employees refrain from membership in a labor union as a condition of employment.
The Supreme Court, on a 6-2 decision, held that the Erdman Act was unconstitutional, because it unjustly violated the due process clause of the Fifth Amendment, which guaranteed freedom of contract and property rights. Furthermore, the court established that Congress' control over interstate commerce did not extend to membership in trade unions. The decision reflects the consistently pro-business slant that the Court took prior to 1910.
For further reading:
The Case
The Decision