This page is a pointer to mark pages that have been deleted.
When the page starts with a link to DeletedPage, it is considered "marked for deletion", and will eventually be expired. Pages are marked for deletion when they are WikiSpam, if they are obviously a test page, or off-topic. If you believe that a page has been erroneously marked for deletion, simply remove the tag and possibly change the page such that it is more obvious why it is on-topic. If your page was a test, consider using the SandBox for testing.
To delete a page, insert the word DeletedPage on a line by itself before any content. You can write anything else you want on the page; you probably should write at least a word or five explaining why (even if it's obvious to you). A useful idiom looks something like:
DeletedPage: This page was WikiSpam.Pages tagged with DeletedPage are deleted after a couple of weeks (actually, the expiry timeout of the [KeptPages]) of no edit activity. In practice, this usually means after someone important remembers to do maintenance. Pages that are deleted are permanently lost. They are not restorable from any backups.