A more thorough explanation:

The Bureau of Diplomatic Security (DS) is a part of the U.S. Department of State. Its main job is to create security plans for American diplomats and interests all over the world. DS also manages the Diplomatic Courier Service, which is responsible for transporting classified documents and materials.

For example, if a U.S. ambassador is traveling to a foreign country, DS will work to make sure they are safe and secure. They might provide security guards, armored vehicles, or other measures to protect the ambassador. DS also makes sure that any important documents or materials the ambassador needs are transported safely and securely.

Another example is if a U.S. embassy is under threat from a terrorist group. DS would work to create a security plan to protect the embassy and the people inside. They might install security cameras, build barriers, or take other steps to prevent an attack.