This is a case that was decided in 1952. It has to do with separation of powers and the limits of presidential power.
In 1952, the Korean War was in full swing. The US military was heavily engaged in the conflict and needed war materiel. At the same time, the United Steelworkers of America were engaged in a labor dispute with the Youngstown Company. Unable to get the company to meet its demands, the union decided to go on strike.
President Truman felt that this strike would be adverse to the national interest. He felt that it would disrupt the supply of steel that was necessary for war production. For this reason, he issued an executive order authorizing the federal government to seize the steel mills and make sure that they kept running. The Youngstown Company filed suit alleging that this was an improper use of presidential power.