This is the first screen you see after you entered your turn in the game loop screen and clicked through any reports or messages there. It is the screen in which you can inspect the map and units and give a number of orders. In some cases subwindows or other screens are opened, but that doesnt matter much from the flow perspective in studying the rules.
Basicly the player can:
* Execute an order. See orders
* End the turn of the regime. And return to the Game Loop Screen