5 members online 1 hidden

[fraZ0R]

What does this mean? How can a member be hidden? Is it an admin? 

Edit: also three guests are doing unknown action currently
wat

[Dav999]

Members can set themselves as hidden in their profile, in that case their name won't show up in who's online, but they'll be included in the count of hidden members instead.

"Unknown action" doesn't mean much. It's just the who's online list might not account for every conceivable page in the system, the forum's developers might've forgotten to add an obscure page to it, or something was added by a modification.