Your card sits in the phone and the shop is handed a one-time number instead of your real one. That substitution is why paying by phone is safer than typing the card in.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a purse that lives in your pocket computer.
For example
You tap the phone at the register. The shop is handed a one-time number, and never sees the long number on your actual card.
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.