A proprietary, controversial encoding once promoted as delivering studio-master quality in a smaller file — now largely receding from streaming.
MQA was marketed as a way to deliver "studio master" sound in a stream-friendly file, using a "folded" format that a compatible DAC could "unfold." It was always divisive: critics argued it was a lossy, licence-fee-bearing format solving a problem high-bandwidth streaming had already solved, and that its claimed benefits didn't hold up to measurement.
Since Tidal moved to lossless FLAC hi-res and MQA's parent company entered administration, MQA's relevance has faded fast. We mention it mainly so you can recognise it — and so you know that you don't need an MQA DAC to get excellent streaming sound today.