There’s a map that shows where every file on your disk is. Doing a regular trash just erases the map pointer for that file, but the 1s and 0s that the file is made of are still sitting there on your disk. Secure erase writes 0s into that area on the disk, so even if you knew where that file used to be located on the disk, now you’ll just find 0s there, instead of finding the 1s and 0s of the old file.