The green "Check" listing tells you what setting you should check or have ticked. The red "Uncheck" tells you what should be uncheck or have unticked.

General:

1.  Go to Admin -> Configuration -> Features and Options -> General. Check: Enable who's online list - allows you to see who is doing what and where. Track daily statistics - tells you stats about your users. Track daily page views (must have stats enabled) - lets you know how many users were viewing specific pages.

(Optional) Uncheck: Allow users to edit their displayed name - you can keep a better track of who is who if they keep the same user name. Let them request it and keep a log of it somewhere so you and your staff know who is who. I suggest creating a "Changelog - Users" either in the admin or staff boards and keep it updated. Allow non-administrators to hide their online status - don't let others hide their online status or hide from others view. Admins can still see hidden users. (Optional)

Layout:

1.  Go to Admin -> Configuration -> Features and Options -> Layout. Check: Show online/offline in posts and PMs - You can still see if someone is online by viewing any of their posts, if they have any. This seems more accurate than viewing Who's Online.

Updated - Sept. 07, 2011