A file format. In this case, one that was proprietary and whose main supporter is in administration (edit: or was now bought up, I am not following this in detail), i.e. MQA, which Tidal is replacing with FLAC.
It’s not so much a Roon issue but a Tidal issue. If you are a Tidal subscriber, you probably know. And the release notes of the Roon update have some more detail. The Tidal support pages have more - I don’t know but I guess they may have sent marketing emails when they announced their new subscription options.