Okay kiddo, so do you know what it means to have ancestors? They are the people who came before you in your family tree.
Now, American ancestry means that someone's ancestors come from different parts of the world and eventually settled in America. This can be a really long time ago, like hundreds of years, or more recently, like in the last few decades.
For example, let's say someone's great-great-great-grandparents came from Italy to America in the 1800s. Those Italian ancestors are a part of their American ancestry.
People in America can have all kinds of ancestry. Some have ancestors from Europe, Africa, Asia, South America, and many other places. This is what makes America such a diverse and interesting place!