Apps keep copies of what they already fetched so they load faster. Clearing it frees space and fixes odd behavior — your actual data stays.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
throwing out the scrap paper, keeping the ledger.
For example
An app is behaving oddly and taking up a lot of room. You clear its saved bits and it works again. None of your photos or messages went anywhere.
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.