This is the Subpage List page template. You need to apply it to a parent page and it will list its sub pages as seen below. This text is the actual page content, above is the page title. In this case I am using it to list all available page templates. The Subpagelist page template will not be shown in the list below since it is the parent page. All other page templates have been applied to child pages of this parent page. I think you get it, right?