A more thorough explanation:

The Judiciary Act of 1789 is a federal law that established the lower federal courts and other functions of the federal judiciary.

The Constitution of the United States provides that the "judicial power of the United States shall be vested in one Supreme Court, and such inferior courts" as Congress sees fit to establish. However, the Constitution does not specify how many justices should sit on the Supreme Court or the structure of federal courts.

The Judiciary Act of 1789 filled this gap by providing that "the supreme court of the United States shall consist of a chief justice and five associate justices." The Act also created federal District Courts and a Circuit Court, which would hear appeals from the district courts and would become the Courts of Appeals.

For example, under the Act, the District Courts had jurisdiction over serious federal crimes, civil cases over $500 involving diversity jurisdiction or the United States as a party, and admiralty cases.

The Act also created the office of the U.S. Attorney General, and for each federal district, the office of United States Attorney and United States Marshal.

Overall, the Judiciary Act of 1789 established the basic structure of the federal court system and defined the jurisdiction of the federal courts.