Google wants you to feel more connected to your content. Android 4.4 KitKat includes native support for fullscreen apps, hiding the status bar and navigation buttons in apps that have the feature enabled.
Select apps may already support fullscreen mode, but many more will have to be updated in order to make use of the feature. You can see fullscreen mode in action with apps like Google Books, but you'll probably notice and appreciate it more for games. With the navigation buttons hidden, you don't have to worry about accidentally hitting "Home" during a particularly intense Candy Crush session.
Before you panic that you'll be trapped in an app forever, know that you can easily access the status bar by swiping down from the top edge of the screen and the navigation buttons by swiping up from the bottom of the screen. You can still accidentally exit out of an app, but it'll be a lot harder than when these parts of the OS were left exposed at all times.
It'll be a while before more apps take advantage of fullscreen mode, especially since such a small portion of the population has access to Android KitKat right now. You can expect it to become more common over the coming year, as more phones update to the latest version of Android.
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