This page was greated by clicking on the Add Page link at the top of the Playground page.  That produces a "child" page and makes the original page a "parent."  You can change the parent of a child page after the page has been created, an interesting biological conundrum.

You can see the hierarchy in the breadcrumbs at the top of the screen.  Once a page has a child, you can see the hierarchy at the bottom of the page. 

If you use the Add Page link from here, the page created will have this page as its parent.  If you go back to the Playground (use the breadcrumb links at the top of the screen) and create it there, it will be a sibling to this page.  Try it out!