From the Finder, when you delete a file and empty the Bin, the file is not really deleted. Instead, only the directory entry for the file is removed, but the file's data remains on the drive.
From the Erasing pane in the Files toolbar item, this option permanently erases the selected file or the folder and its contents.
The Secure Delete (3 passes) option overwrites every byte of deleted contents three times (with 0xff, 0x00, and 0xff again).