A guest is a visitor on your forum that is not currently logged into a member account. For example, this could be someone new to the forum checking it out or, an existing member that hasn't logged in yet. It could also be a search engine crawling the forum so that your forum can be listed in search results.
Depending on the permissions you've set, guests can read topics on your forum and even participate by posting. If you would like to prevent guests from posting on your forum, visit your Administration Area and click the the "Modify Board" link. For each board that you want to restrict posting on, make sure that "Posting Restrictions" for each board are set to "Registered Members" in the options. If you would like to prevent guests from viewing topics on this board, set the "View Board" option to "Registered Members".
Another method to prevent guests from viewing the content of your forum is by setting the "Guests Must Login" option found in the "General Settings" area of your Administration Area. With this option enabled, anyone that is not logged into a member account on the forum will be required to login or register before continuing into your forum.