When you're working on something important you might want to delay responding to incoming messages. However, when you're done working, will you remember them? If you're anything like me, you'll have a lot of buffers in your Emacs session, and a jabber chat buffer can easily get lost.
When you type M-x jabber-activity-mode Emacs starts keeping track of the buddies which have messages you since last you visited their buffer, and will display them in mode line. As soon as you visit their buffer they disappear from the mode line, indicating that you've read their message.
If your mode line fills over because of these notifications, you can
customize jabber-activity-make-strings to shorten them to the
shortest possibly unamiguous form.
If you try to exit Emacs while you still have unread messages, you
will be notified and asked about this. If you don't like that, set
jabber-activity-query-unread to nil.