From Online Manual
At the end of the forum index, there is a section called Users Online. This is a list of users who have recently used the forum. This feature can be enabled or disabled, and the definition of "recent" changed from Features and Options in the admin panel.
This list is ordered by most recent activity. Members who belong to color-coded membergroups (such as administrator or global moderator) will be color-coded on this list. Administrators, are shown in red and global moderators are shown in blue by default. Regular users appear in the default color used by the forum theme.
To learn more about what users are doing, select the Users Online icon or the link below the heading to display the Who's Online page. This page is a list of everyone (guests, members, and hidden) currently browsing the forum along with the last thing they tried to do on the forum. For example, this is the Who's Online page for simplemachines.org.
In the Who's Online section, you can occasionally see some users (or even guests) whose action is displayed as Unknown Action. You may find this strange and suspicious but it is not. This action is displayed when you have a modification installed on your forum and this modification does not declare itself properly, meaning that the forum does not recognise what is the user doing.
Nothing, or nothing you can see...
It is very common to see this action displayed when viewing the Who's Online list. In most of cases, this will happen if you are a regular user because you do not have permissions to see what some other users are doing, for example, if they are posting in a board that you cannot see. As an administrador, you may also see this action displayed for regular users. In this case, it means that users are trying to access a deleted message or topic or other pages that no longer exist.
Why do I see my guests viewing threads in Who's Online when they cannot view the forum?
Is it possible to prevent guests from viewing the forum by unchecking the Allow guests to browse the forum in Admin > Configuration > Features and Options > General. If you uncheck this option, guests will not have access to any part of the forum, and they will be forced to log in or register.
Even with this feature enabled, some guests will be viewing threads in the Who's Online section. These guests are . Spiders are internet bots used for web indexing (such as Google or Yahoo) and they will be accessing your threads to index them. For more details about spiders and how to change the settings relating to them, please see http://wiki.simplemachines.org/smf/SMF2.0:Search_engines.