This option makes it nearly impossible for the drive's heads to be used for reading any residual magnetism patterns and recover files. This option writes zeros to the unused portions of the drive and overwrites any previously deleted files. To prevent this from happening, one option that Apple has offered in OS X is to erase the free disk space using Disk Utility. While this offers a quick way to delete files from the drive, the data is technically still intact on disk and thereby leaves open the possibility for recovery using special software. Conventionally when files are deleted from storage device, the system only removes their entries from the file system's directory, thereby flagging blocks containing the data as being free to use. Erasing free disk space is an easy (albeit not quick) way to help prevent the recovery of files you have erased from your system.
