When you go to create an account or to change your password, you enter a new password. If the new password you’ve chosen matches any password currently used by any user on the system, you’re told you need to choose another password.
Because I work in information security, I deal with passwords a lot. There is a really simple guide to making good passwords: longer is stronger. If you remember nothing else, simply remember longer is stronger.
I moved all my data from one mac laptop to another. I’ve done this many times with Time Machine, and it always works. But I often forget a few steps. This is my attempt at jotting down the things I forget to do, so I don’t forget the next time.