Roon is a music management and playback software that organizes your music library and streams it to many devices. It's designed to provide high-fidelity audio and a rich experience for exploring your music.
That's what Google says.
I tried Roon and it is very slick. I really liked it. But, what I think I learned is that it's very processor intensive and you need hardware optimized for Roon if you want optimized sound quality. I don't have that, so I'm using the Sense app from Innuos. It works really well and it sounds better with my hardware.