The second window, the Mixer Window, can be viewed by going to the main menu and clicking Window > Mixer.
The main function of the Mixer Window is to display all the Mixer Strips for the various Tracks side by side for easy reference. This window is mainly used during the Mixing process, and it also provides access Plugins and other Routing features. See the chapters on Understanding Routing, Mixing and Using Plugins for details.
This window also contains a listing of the available Mixer Strips in the upper left hand corner. The check boxes in this area can be used to view and hide the Mixer Strips of the different Tracks. Finally, there is an area for managing Groups.