A second proof on top of your password — usually a code on the phone in your pocket. A thief who steals the password still cannot get in without the phone.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a bank locker needing your key and the manager's.
For example
Somebody in another country types your password correctly. Your phone buzzes with a code. They do not have your phone, so they do not get in.
This is one card out of 275.
The hundred words you meet most are free — the whole set, animated, in one file that works with no internet.
Volume 1 has all 275 in fifteen chapters, with Staying Safe the largest chapter on purpose.