It is playing of music over the network; where you have one common library which can be used by multiple people (a family for example) on different devices with different outputs (you can also group a number of outputs to play the same music). You can control all outputs from any device (windows, macos, linux; android; ios). It is a very advanced airplay 2; but just the audiophile version.
I would like the Vero 4K to work as an audio device (output device) by supporting the RAAT protocol; everything else is already handled by Roon.
More information can be found here:
Edit:
It also got streaming integration with Tidal (and a few more is popping up eventually). So for me with Tidal subscription; I have a great mix between my content stored on NAS (or PC); and Tidal. It is the same. With a bunch of meta-data collected; and it can point you to similar music both in your local library and in Tidal library. I think Spotify though this would break their business; so they blocked it. But others bit-perfect options are coming.
By the way; the goal of Roon is bit-perfect playoff on any device.