The few letters after the dot decide what can open a file. Renaming them converts nothing — the contents stay exactly as they were, only now they are mislabeled.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
the language a document is written in.
For example
A file will not open and somebody says to rename the ending. You do, and it still will not open — because renaming the label changed nothing inside.
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.