Starts the device with only its own essentials loaded. If the trouble disappears in safe mode, something you installed is causing it and the machine itself is fine.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
switching every breaker off, then back on one at a time.
For example
The laptop keeps freezing. You start it a special way, with only its own basic parts running, and it behaves perfectly — so something you added is the problem.
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.