Split View allows you to use Full Screen mode but with two apps instead of just one. You start Split View by holding the green button on one window and then choosing the left or right side. Then you click on another app to place it on the other side. The Split View screen acts as a Desktop and can be accessed from Mission Control. You can exit Split View by using the green button again on one of the two apps, which leaves the other app behind in Full Screen mode. But there is a way to exit Split View and return both apps to normal mode at the same time.