Public - Anyone may enter, unless you disable Guest access. If you disable Guest access, your message board will be inaccessible to anyone not registered with the main forums system. They will get a login screen when attempting to enter your forum, and a message telling them your forum doesn't allow guest access.

Password Protected - You set a password you give it to anyone you want to have access to your forum. They will log into the main system, then they will need to log into your forum.

Restricted - You add people to your forum's member list in your forum's Admin area, then they can enter just like a public forum. Anyone not on the list will get a page telling them they're not a member of your forum. You'll see them on your visitor log to let you know they tried to visit. This lets you decide if you'd like to let them in or not. If you do, add them to your forum's member list. If not, don't.

Forum owners and administrators find these controls in Admin > Forum Access.