A major under-the-hood feature introduced in Android 4.4 KitKat was a new runtime called ART (short for Android Runtime), an alternative to Android's default Dalvik runtime for apps that has been use since the first version of the OS. ART, currently in the experimental stage, changes the way apps are installed and stored on a device. With Dalvik, each time you run an app, the part of the app's code that is required for its execution is compiled into machine code at the very moment. ART, on the other hand, compiles the necessary code and saves it on the device during the app's installation, removing the need for the code to be compiled whenever an app is launched, hence boosting app startup times and also (in some cases) battery life.
ART is part of the default KitKat update, but Samsung (and mostly every other manufacturer except Google) didn't include it in the KitKat update for its existing devices, which put off many as ART does offer advantages even in its experimental stages. However, it looks like ART will be an option on the Galaxy S5, according to a screenshot posted online, which will make the tech-savvy folks who want to get the most out of their smartphone happy. It's not exactly unexpected, as Samsung probably found it easier to implement ART in the Galaxy S5 as it must have developed the device's firmware from scratch with KitKat as a base instead of using a previously available firmware, but it's nonetheless a good thing.
You can find more details on how ART works here. Remember, work on ART is still ongoing, but it's expected to be the default runtime from the next major version of Android, though many app developers are already adding ART support to their apps.