In the olden days, when you wanted to replace your hard drive with a bigger one, you’d run a “secure erase” on it to completely remove any personal data. This would write zeros to the entire disk, overwriting anything already there.
But now, thanks to advances in storage tech, this no longer does the trick. (Not that you can change your own Mac SSDs now anyway.) The new secure-erase, says Apple, is to just encrypt your disk.