[CarCdr]

Decent examples of its use can be found in forumdisplay.php or threadrate.php. It's a vB function that is used to store an array of information by key, like forumid or threadid.

As for checking when a forum was last viewed, you should see that in forumdisplay.php. The cookie may not be set, so you'd need something along the lines of:

$last_view = fetch_bbarray_cookie('forum_view', $foruminfo['forumid']);
$last_view = max($last_view, $bbuserinfo['lastvisit']);