It's a built in feature of kitkat. When you watch a video or something that has access to the " full screen. " It will hide the soft keys and the notifications.

Just go to you tube, load up a video and put it landscape mode, you'll notice where the staus bar or icons would be and the soft keys are now gone. That is immersive mode.

 

Yeah no. Some roms have that feature built in. I'm using Cataclysm and it has it.

Basically your status bar and nav bar are hidden unless you swipe down from notifications or swipe up from nav bar

 

Immersive mode off


Immersive mode on


Immersive mode on (once you swipe to see the bars)

 

As this isn't the All Things Root sub, we can assume he meant the built-in immersive mode
Android KitKat | Android Developers

 

It is built in, and the app you are using has to be coded in order to take advantage of it.

See here for more info.