A more thorough explanation:

The Garmon Doctrine is a legal principle in labor law that prohibits state and local governments from regulating certain activities related to labor unions and management-union relations. This doctrine is based on the idea that federal law should take precedence over state law in certain situations.

For example, if a state law conflicts with the National Labor Relations Act (NLRA), which governs the rights of employees to organize and bargain collectively, the Garmon Doctrine would prevent the state from enforcing that law. Similarly, if a state law prohibits an activity that is considered an unfair labor practice under the NLRA, the Garmon Doctrine would prevent the state from enforcing that law as well.

The Garmon Doctrine was established by the U.S. Supreme Court in the case of San Diego Bldg. Trades Council v. Garmon (1959). The Court held that state and local governments cannot regulate activities that are either protected or prohibited by the NLRA, as doing so would interfere with the federal government's authority over labor relations.

Another related legal principle is the Machinists preemption, which prohibits state regulation of areas of labor activity that Congress has intentionally left unregulated.