Master Quality Authenticated, an optimization of streaming that delivers a higher quality audio, similar to the one delivered by CDs.

I assume it was a closed source or even patented software.

https://www.makeuseof.com/what-is-tidal-mqa-why-is-it-controversial/

It was a sort-of trivial attribute, like "more bubbles in your shampoo"--a marketing ploy, essentially.

A non open-source format for encoding master quality audio - something that you’d use to share super high quality audio as opposed to an open source format FLAC. #[2]​ and team would know better, but that’s my understanding.

No DRM restrictions, open file format.