From Online Manual
Who's Online is a list of guests and members (if they have chosen to show their online status) recently browsing the forum along with the last thing they tried to do on the forum and when, as well as (optionally) one or more ways to contact the user (including the "personal message: online" icon ) By default, the most recent actions are listed first; click on column headers to change how the list is sorted. Members who belong to color-coded membergroups will be color-coded in this list. To get to 'Who's Online from the board index, select the Users Online icon or the link below it. Anyone with "moderate forum" permission (administrators and global moderators) can see members who have chosen not to show their online status.