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It allows you to add virtual Desktops, to virtually expand the screen size of your computer. With El Capitan Apple added a new feature called Split View. Split View allows you to have two (compatible Split View) apps, side by side.

Is activated by clicking and holding the maximize green circle at the top left edge of any application window.

Drag the window on the left or the right side of the screen. Choose the second app, among those open at the time, to have it shown at the opposite side of the main window.

That's Mission Control -> Use Mission Control on your Mac - Apple Support. Mission Control lets you switch between different windows quickly, and also lets you create and use virtual desktops. This is very useful if you need to run a lot of applications at the same time.