What exactly is ROON?

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.

I was going to write it out but honestly it's all here:

https://roon.app/en/how-roon-works

While it’s ultimately software that can run on your computer, it runs best on Roon’s own server, the Nucleus.

and you need hardware optimized for Roon

I had it for a while but didn't really use it much. I confess I can't sit still for streaming music (or CDs for that matter) for very long before I am distracted and lose interest. 

The people who do like it seem to REALLY like it though so your mileage may vary.

I think if you like reading liner notes you would like Roon.  If you don’t know what liner notes are because your music appreciation evolved with iTunes you can read about them here:

https://www.merriam-webster.com/dictionary/liner%20notes