The bars are not a standard measure and mean different things on different phones. Moving a few steps, or to a window, changes reception more than anything else.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
how well you hear someone calling across a field.
For example
You have one bar by the kitchen wall and four by the window. Same phone, same minute — you simply moved three steps.
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.