When you toss a file into the Trash and then empty it, the file isn’t really gone. With the proper recovery tools you can recover it provided that you haven’t added other files to your Mac that have overwritten the portion of the drive that file once occupied. While you can’t go back and target specific deleted files for “no really, I want you totally gone” erasure, you can erase the “free” space on your Mac’s hard drive. The default, Zero Out Deleted Files, writes zeros over the drive’s free space.”]
Source: https://www.csoonline.com/article/2131400/how-to-securely-delete-files.html