ART? simply the new runtime that made it’s debut in 4.4.
It’s still experimental so it’s not the default runtime, and can cause a bit of problems. Some apps just do not like it, causing crashing/slow apps at times. Be aware of that before testing.
First: you need to enable Developer options. With that enabled, go to Settings > Developer options, tap Select runtime where you select ART. After this the phone must restart, and this takes AGES since all apps have to be “adjusted” to ART. That’s it. Changing back is of course just as easy.