This is a sample of our Side Pop-open links. We call them this because selecting a sub-page of one of the "main" pages on the site causes that "main" link to pop open and display the sublinks.
This page ("Side Pop-open Links") is a sub-page or child page nested several levels down. It is a child of "Website Features" so that is the main link that has been "popped" open. But since we are deeper still, you see that our other "ancestor" pages "Navigation" and "Sample Tree" have also been "popped" open.
To see how this works as a drill down feature, click on the "Pop-open + sub-pages" link and notice how the links change when a page with yet more sub-pages is opened. Go back and forth if you missed it.