App store link: 
This is an app written by Apple. It does a very simple thing. Over your wifi connection it controls your iTunes. You can browse the music/video library on your computer and play it. Just like you can play the part of your library that’s on your phone. See the album art. Everything. The great thing about it being Wifi is you don’t have to be within line of sight to access it. You can be in another room. Or downstairs. As long as your library and your phone are on the same wifi network it will work. Very useful, even if it is a pretty simple concept.