Samsung has a pretty good track record of releasing source code for its flagship Android devices, something that's a requirement of a member of the Open Handset Alliance). They sometimes even release key parts of the source before a device is released or before an OS update is pushed out.
This time, it's the kernel source code of Android 4.4 KitKat for the Galaxy S4 Google Play Edition (model number GT-I9505G). Samsung hasn't announced a time frame yet for this device to get KitKat, but now that the kernel source is available, it shouldn't be too much longer.
This kernel source is available on Samsung's Open Source Release Center. The bottom item in the list is clearly labeled KK for Kit Kat. We can likely expect an official announcement about the availability of the full update pretty soon.
For those who may not know, the kernel is an essential part of all operating systems. It's essentially the code that lets software talk to the hardware. If an app wants to use WiFi, or adjust screen brightness, or things like that, the app talks to the kernel which makes it happen. The OS itself talks to the kernel constantly, too, doing things like speeding up the CPU for games, or scanning for nearby Bluetooth devices. The kernel is also responsible for memory management and the mobile cellular radio.
This is why its common for the root community to use alternate kernels. Developers can take the kernel source from Samsung and tweak it to do things it wasn't originally intended to do, thereby gaining extra performance out of their devices. Common kernel tweaks include increasing WiFi speed, adjusting camera image compression, and improving battery life by tweaking performance settings.
It's worth noting that both the Galaxy S4 Google Play Edition and the international LTE Galaxy S4 both share the same hardware, meaning the talented hackers could take this kernel source and use it to potentially bring Kit Kat or other goodies to the international LTE S4. The hacking community might be able to get a lot of use out of this kernel source.
For average users, this isn't all that exciting. Source code isn't directly useful to most users, but even if you're not someone who can take advantage of it, you can at least get excited that the official KitKat update for the Galaxy S4 Google Play Edition shouldn't be far off as it's already hit the HTC One Google Play Edition.