From Joomla! Documentation
To open the Menu Dashboard select the dashboard icon to the right of the Menu item in the Administrator menu. The default dashboard contains two panels, one for Menus & Menu Items and one for items in individual Site menus. Neither can be edited or unpublished.
You can have as many Site and Administrator menus as you like in almost any position you like. This feature is often used to show different menus to different User Groups after login.