A more thorough explanation:

Standard Oil Co. of New Jersey v. United States (1911) was a case heard by the U.S. Supreme Court. The case found that Standard Oil Company, a major oil conglomerate in the early 20th century, violated the Sherman Antitrust Act through anticompetitive actions, specifically forming a monopoly. The court ordered that the company be geographically split.

The Standard Oil Company of New Jersey was a holding company owned by the Rockefeller family. The family organized their oil empire by creating holding companies in many of the jurisdictions in which they operated. In total, the Rockefeller family and their holding companies controlled almost the entire petroleum market in the U.S. To further their control over the petroleum market, the Standard Oil Company of New Jersey had acquired nearly all of the oil refining companies in the United States.

The United States brought suit against the Standard Oil Company of New Jersey, alleging that it violated the Sherman Antitrust Act because its acquisitions were an undue restraint of trade. The Court ruled that Congress had the power to pass the Sherman Antitrust Act under the Commerce Clause of the Constitution. It then ruled that “restraint of trade” included monopolistic behavior, and only unduly restrained trade if it led to higher prices, reduced output, or reduced quality. The Court found that Standard Oil of New Jersey’s actions led to these consequences and therefore violated the Sherman Antitrust Act.

For example, if a company owned all the gas stations in a town and raised prices significantly, that would be an example of a violation of the Sherman Antitrust Act. The company's actions would be considered anticompetitive and would lead to higher prices for consumers.