This is the main UI frame where your current view is loaded.
When you select a view in the bottom menu it will load the view in this center area.
A view can be something like the Dashboard, the Map, Roadbook, Speedometer, Control Station, etc.
The available views (accessible in the bottom menu) may vary according to your purchased items.
The Dashboard, Speedometer, Applications, Control Station, Global Settings are standard, included for free, but views like the Map and Roadbook need to be purchased first.
Each view will be covered in different guides.