It just means that DU is correcting the ownership (and maybe permission flags) of those files - if you go into Terminal and type "ls -l" (minus quotes, then hit 'return'), you'll see the various permissions and ownership info for some of your files. Correcting the UID means in essence that it's correcting ownership.
Edit: the '/private/var/at' files end up being owned by "daemon", not "root", FWIW.