Team Members are your trusted collaborators: these people have the ability to make changes to your boards, add content, modify content, etc.
And, most importantly, they can create new boards in your accounts. (But, don't worry: you are still the Account Owner, which means you have final control over everything that happens in your account.)
If someone is a Team Member on several boards in your account, that person is counted only once as being part of your Account Team.
Here's an example: you have three boards, and some people are working as Team Members, and some people as Visitors: we are showing the Team Member faces in red squares, and the Visitors in circles.
Everyone who is a Team Member, on any of your boards, is part of your Account Team.
Visitors have read-only access to your boards, which means they can see your board's cards, canvases and other attachments.
They cannot view any of your board's chat conversations; they cannot be assigned any items; and they don't count when it comes to your account team size.
It's easy to assign tasks to people who are already part of your Account Team: you can add them to board teams without having to send invitations each time you set up a new board.