The iPhone 5's longer screen is great, but not all your apps have been updated yet to support the extra pixels. The result is a boxed app, which isn't really an issue until you invoke the keyboard and it's halfway up the screen. FullForce for the iPhone 5 fixes that.
It forces the apps to fill the screen, just like FullForce for iPad, creating an iPhone 5-optimized look and feel without the developers having to do anything. You have the option to stretch the app or not on launch, just in case it causes problems.