Immersive (FullScreen) mode is a new, handy tool that has been released in Android 4.4 KitKat. This mode hides all the object that overlap the current activity - hides notification and soft keys bar. This mode, however, was not available for all the apps but now you can enable it with this awesome module for Xposed Framework!
As you can see on the picture above, the immersive (fullscreen) mode is enabled everywhere - even in the Google Play Store. However, if you want, you can always access the notification and softkeys bar by swyping your finger from the top/bottom of the screen.
This mode is for XposedFramework, therefore be sure you have this framework installed and you enabled this module in case you want to use it.