Deleted

http://www.piriform.com/docs/ccleaner/using-ccleaner/wiping-free-disk-space

"Deleting free space" delete what stays on your hard drive after deleting data.
When you delete a file from your drive, it is not actually deleted, the space he take is just marked as "empty", though it isn't
"Deleting" the free space replace this data by true empty data. It's mostly used in computer security, it's useless for normal computer use.

Vaba ruumi kustutamine hävitab jäljed varasemast kasutusest, salasõnad ja muu. Kasutatakse, kui antakse arvuti taaskasutusse vms. See ei muuda vaba ruumi mahtu, lihtsalt kustutab jäljed!

In layman terms, when you delete files on your hard drive they aren't really "deleted" yet. What cleaning free space does is actually get rid of the file from your drive permanently so that it can't be retrieved using file recovery software or other methods of recovery.