When you open a file, if you start the path with /sudo:root@localhost: you'll get a prompt for your user's sudo password. After entering a correct password, you enter the file path.
So /sudo:root@localhost:/some/protected/file is a more convenient way to edit the file, rather than quitting and running sudo emacs /some/protected/file.
You can also use /sudo: in bookmarks. I've got a bookmark called "hosts" which jumps to /sudo:root@localhost:/etc/hosts