Android RunTime. It's basically a (supposedly) more efficient virtual machine than the Dalvik machine currently running in Android (and the apps have to be worked on in the phone to be able to run in ART).
Is it better? Some people who have tried it say yes, some say no.
Best not to mess with it. Prior to Android L versions, ART is experimental. Also, the benefits in terms of performance are not that big. YouTube has side by side comparisons to show the difference.