Visitors can read all the cards on a board, as well as all the content attached to them: files and Web links.
They cannot, however, read any of the chat that's part of a board.
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.