About the term "top-level-browsing-context"

In the [spec](https://html.spec.whatwg.org/#top-level-browsing-context):

> A browsing context that is not a nested browsing context has no parent browsing context, and is the top-level browsing context of all the browsing contexts for which it is an ancestor browsing context.

I think I can't understand what it means very well,could you give some little example?Thanks a lot.