That's an indication of how many are viewing some part of the message board, but aren't logged in. They may be a registered member, but if they haven't entered their username and password, they will show up as Guest but in Guest status, they can only view. Just as when I invited you to register, prior to registering, whenever you browsed the board, you showed up as one of the Guests.
You can confirm that by looking at the tally at the bottom, signing out and you'll see your name disappear and an additional Guest tally will appear.
