A more thorough explanation:

Definition: Nationality theory is a jurisdictional principle that states that citizens are subject to the laws of their country, no matter where they are.

Example: If a person is a citizen of the United States, they are subject to U.S. laws even if they are traveling or living in another country.

Explanation: Nationality theory means that a person's citizenship determines which laws they must follow. For example, if a U.S. citizen commits a crime while traveling in another country, they can still be prosecuted under U.S. law. This is because they are subject to the laws of their home country, no matter where they are in the world.