So imagine you live in a big house with your family, and your family has to pay for all kinds of things to keep the house running smoothly like buying groceries, fixing broken things, and paying for electricity and water. This is kind of like how a country works, except the country has millions of people living in it and lots of different things to pay for like schools, roads, and hospitals.
But what about people who don't actually live in the country, like your friend who moved to a different house in a different neighborhood? They still have to pay for some things even though they don't use them, like if they still want to watch movies in the movie theater. That's kind of how it works for nonresident Americans - they don't actually live in the United States, but they might still need to pay some money to the government.
The government does this to make sure that everyone who benefits from being an American, even if they don't live in the country right now, still contributes a little bit to help cover the costs of running the country. Depending on how much money they make and where their money comes from, nonresident Americans might need to file a special kind of tax return and pay some money to the government every year.
It might seem a little bit confusing, but just remember that being an American means being part of a big group of people who all work together to keep the country running, even if some of them live far away.