This is the Main Window which is in front while running. You can put it into the taskbar to see only the Preview Window on your desktop. From this window you start most activities.
Here you see the current video source. You can select always on top for this window, to see at every moment what's happening.
You can spread the Preview Window to fullscreen if you like.
You see only the Preview Window on your screen with a black border around.
Take a look at the last taken picture.