How many dots make up the picture. Past a point your eyes cannot tell at arm's length — brightness matters far more to how a screen actually feels than pixel count does.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
threads per inch in cloth.
For example
Two televisions show the same movie. Standing close, one looks sharper. Standing back on the sofa, you cannot tell them apart at all.
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.