The app does not need to be open. Its company sends a message to your device whenever it chooses — which is why the setting is worth visiting.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a tap on the shoulder from a shop you once visited.
For example
A shop you visited once last year taps you on the shoulder at nine at night. You never opened their app; they opened yours.
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.