ART stands for the Android RunTime. It's a new engine, basically, for running applications. The old one was called "Dalvik".
Dalvik would read the code as the app was running and compile the bits you needed as you needed them. This could show up as a slight delay while running an app, or longer app startup times. However, "installing" an app was a matter of putting the source code into a folder, which is very, very fast.
ART downloads the source code package and compiles it at install time. This means less time to run an app, but more time doing the one-time step of installing it. So, as you can guess, installation of apps takes considerably longer (but once compiled, apps should run faster with less CPU consumption, so you'll get better performance and slightly better battery life - in theory at least).