A more thorough explanation:

The United States Marshals Service (USMS) is a federal law enforcement agency that operates under the Department of Justice. Its primary responsibility is to ensure the effective operation of the federal judiciary. Each federal district has a U.S. Marshal appointed by the President.

The USMS has several duties, as outlined in Title 28, Section 566 of the U.S. Code. Its primary duty is to provide security and enforce all orders of the federal courts. This includes providing security at federal courthouses and ensuring court sessions proceed in an orderly fashion. U.S. Marshals also execute all lawful writs, process, and orders, and command all necessary assistance to execute their duties.

The USMS has the broadest arrest authority among all federal law enforcement agencies. They may arrest individuals who commit a crime in their presence without a warrant. The USMS is also authorized to investigate fugitive matters and often works with local law enforcement agencies to apprehend fugitives. Additionally, U.S. Marshals are authorized to protect federal jurists, court officers, witnesses, and other threatened persons in the interests of justice.

• A U.S. Marshal provides security at a federal courthouse and screens visitors to ensure they do not bring prohibited items into the building.
• A U.S. Marshal escorts the jury to and from the jury room during a court session to ensure their safety.
• A U.S. Marshal arrests an individual who commits a crime in their presence without a warrant.
• The USMS works with local law enforcement agencies to apprehend a fugitive who has fled across state lines.
• A U.S. Marshal provides protection to a federal judge who has received threats against their life.

These examples illustrate the various duties of the USMS, including providing security at federal courthouses, executing lawful orders, arresting individuals who commit crimes, investigating fugitive matters, and protecting federal jurists, court officers, witnesses, and other threatened persons.