The MFT is the Master File Table which contains the identifying information used by the file system to manage file storage. When files are deleted, details like the location of the file's prior contents (and sometimes a small component of the file's actual contents) remain for an indeterminate period of time in the MFT.
Much like the file data locations themselves, the MFT entries are released and become available for re-use, but until they get used again for other files, the information they contain can be accessed to gain information about the deleted files. This is the first line of approach used by file recovery programs, so in some situations it could be viewed as a security risk, enabling private or sensitive information to possibly be recovered after file deletion.
So wiping the MFT free space will erase any remaining information in the previously used MFT records that are awaiting re-use. If you wish to be extra secure about deleted files being completely gone, then you can choose the MFT free space wipe. But keep in mind it will make recovery of an accidentally deleted file far more difficult after applying the action.
Edited by Platypus, 26 November 2014 - 03:04 AM.