It fetches pages from other computers and draws them for you. It is also where the address and the padlock live — your only view of who you are really talking to.
The drawing that proves the sentence above. It is the same card that is in the book.
Think of it as
a window you carry from building to building.
For example
You want to read a page. Something on your machine goes and fetches it, then draws it for you. That something is the window you look through.
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.