Alice knows Bob and Carol, and can vouch for them. Carol knows Alice but not Bob. Since Alice has vouched for Bob, Carol is more likely to trust that the user "Bob" is really Bob.
By "knows" do you mean "follows"?
Someone with a built up report. Could be a repeat customer, a friend, a buisiness partner. Could be a miner who's mined honestly since they came online.
That’s an important point I mention as follow does not imply trust and vice versa. To vouch for someone, you would have firsthand knowledge of who they are but not necessarily follow
