Visitors can read all the cards on a board, as well as all the content and chat attached to them: this includes files and Web links.
And, of course, Visitors can't add new cards, move any cards, or make changes to any Task Board, Scrum Board or Whiteboard.
Visitors don't count in terms of your Account Team size: you can have as many visitors to your boards as you like.
Every project has some stakeholders: customers, senior managers, and other team leads who have an interest in following a board's progress, but are not actually working on any items.
These people make the perfect Visitors: they get a real-time view of what's happening on a project, so you don't have to keep sending them status reports any more, but at the same time they are restricted from making changes that might not be helpful.