Appindicator was first introduced in Ubuntu karmic as a replacement for the Gnome panel applet. It is a small applet to display information from various applications consistently in the panel. It can also be used as a access point to access (and control) the application without having to open the application. if you are running Ubuntu Lucid or Maverick, you should see the messaging menu (the applet that contains Empathy, Evolution and Gwibber icons), which is a good example of an appindicator.