The Galaxy Note 3 introduced a number of new S Pen features, one of them being the Pen Window. This allows you to draw a box on top of whatever you're doing and open up another app inside of it. The list of apps that you could use this feature with wasn't a long one, however, which makes it a less than perfect execution of a cool idea. Once rooted, a user can enable any app to work with the Pen Window feature, making it incalculably more useful.
It's super simple: just go to the original thread over at XDA, copy the APK to your rooted Note 3, install, and go! Check out the video below for a demonstration. The good part starts at around the 2:18 mark.