Lists pages and subpages for a given page, or the top ancestor of the current page and all of the ancestor's descendants
A common web design pattern is to have a primary navigation with the top level pages, and then a separate navigation element on each page that shows only those pages related to a common top-most page. This widget helps to build those secondary navigation elements.
If your page structure was like this:
and you added the widget with its default settings to a sidebar, it would create a list like this:
on any of the pages in the About subtree. Notice that the top-most ancestor, About, is included as the first element of the list.