A set of rules deciding which traffic may pass. Your router already blocks most uninvited traffic — today the danger is far more often something you invited in.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a doorman working from a guest list.
For example
Nothing uninvited gets in through the front. The trouble walked in through a door you opened yourself, holding an attachment.
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.