When you delete a file, OS X removes the reference to that file, but doesn't delete the actual data that made up the file on your hard drive. Over time, this data will be overwritten as OS X writes new files to that area of the drive.
This means that, given the right software, someone could reconstruct all, or parts of files that you've deleted. For privacy and security reasons, you can set CCleaner for Mac to wipe the free areas of your hard disk so that deleted files can never be recovered.
Note: Wiping free space can take several hours.
To wipe your drive using Erase Freespace:
Wipe Free Disk Space limitations
CCleaner for Mac can't wipe every deleted file from your free disk space. There are some limitations, because of the way OS X stores some files. Here are some examples: