You have two choices under that—“Standard” and “Zoomed.”
As you change the setting from the tabs at the top, you can swipe the screen to view how each option looks with different apps. When you’re finished selecting either view, touch the “Set” button at the upper-right, and your iPhone will restart to make the change.
There’s an important key to this feature, though. As iOS 8 notes within the “Display & Brightness” panel shown in my first screenshot, the “Standard” option reveals more content, but the controls are larger with “Zoomed.” This has some pretty serious implications for the 6 Plus; if you pick “Zoomed,” you won’t be able to use some of the fancy screen-rotation features, such as the extra functions on the landscape keyboard…
…the two-paneled layout of Mail and many other apps…
…or the landscape Home screen orientation.
If you’ve got “Zoomed” on, those just won’t work. So you’ve gotta choose: Do you want big bright fancy on-screen controls, or do you want smaller interface elements with more options? And is it a total first-world problem that I’m having difficulty deciding which one I’d like, so I keep switching back and forth? Geez, it’s like Sophie’s Choice.