Picture in Picture
The iPad already supports picture in picture, and now it is coming to your Mac. With macOS Sierra, you can load up a video and pop it out so that it hovers over top of the rest of your screen.
On OS X El Capitan you can already use a window to watch, but it doesn't always stay on top of what you are working on and it doesn't stay in full screen mode.
With this update, you can now keep that video on top of whatever you are working on so that you can watch the game, a movie or a TV show without constantly losing it in the background.