All applications you use on your android device actually run in what’s called a virtual machine, the Dalvik runtime environment. Basically, it’s software that lets you run other software(apks). On the current setup, Dalvik operates as a JIT compiler. JIT is just-in-time, which compiles code as it’s needed to run the app, every time you start it. While it works fairly well, it isn’t the most efficient way to get the job done, and does take up device resources.
Google has now built into it’s newest os, Kitkat, an option to use ART(android run time) instead of Dalvik. You can change to ART under developer’s options in settings. After selecting ART, a reboot is necessary, and then you will see the device optimizing apps. Once complete, it will run more or less as before.
So why switch to ART? Mainly, to take advantage of it’s AOT(ahead-of-time) compiling process. ART will compile apps to machine code when first installed, and after this initialization apps should start up much quicker on subsequent loads. Another benefit is the reduced amount of resources needed to run apps. The eventual aim is to speed up both loading and run times of apps. As a side benefit, Google gets more control over it’s os. It also avoids any potential conflicts with Oracle, which sued(and lost) Google over patent infringement related to Dalvik.