This is an example page. It sits in the tree structure along with your forums and categories. We call this the "node tree", as it's not focused on forums. Right now, you can include categories, forums, pages, and "link forums", which are really just arbitrary redirects.
A page is primarily designed as a way to include custom, mostly static content in your forum. You have full access to HTML and the entire XenForo template syntax, including some default variables (like info for the user that's browsing). It's not designed to rival a CMS or a portal.
Putting it in the node tree automatically gives it the correct breadcrumb structure. More significantly, the page automatically gains access to the node tree's permissions system. You can specify permissions on the page itself or let them inherit from a parent node. If you deny access to a page, all child nodes (of any type) will be denied as well.
While it sits in the node tree, it doesn't have to show up in the hierarchy. All nodes have the option to not show up in the list, but they're still accessible if the URL is known. The breadcrumb will still work even when the page isn't shown in the list.
Have a look at the URL. Notice that there's no ID in it. This is an option for all node types,
though it's only exposed for pages now and is currently available for pages and forums.
You might've also noted the navigation that's on the left here. That's auto-generated, depending on whether you want it to include siblings, children, or neither; if you choose neither, you can still build it manually. I've chosen siblings and children to show it off. :)
Finally, there's some page info in the bottom right, like publishing date and views.
Hopefully you can think of some uses!