Merge suggested (subject already covered on another help page)
Undelete is a special page for wiki administrators to browse the revisions associated with deleted pages (so called deleted revisions).
To view Special:Undelete you need the deletedhistory permission. If you also have the browsearchive permission, you will additionally be able to search for deleted pages. Note that browsearchive does not imply deletedhistory.
The text of the deleted revisions will only be viewable with the deletedtext permission.
Finally, you can restore deleted revisions with the undelete permission. Note that undelete implies deletedtext (as you wouldn't want to restore text you couldn't see.
For completeness, it is perhaps worth mentioning that the undelete permission alone isn't sufficient to restore a deleted page, you additionally need the deletedhistory permission, which, unlike deletedtext, isn't implied by undelete.