MMM is a measuring method with a microphone moved by hand near the listening place : response obtained with this method gives a reliable representation of audibility.
The most complete infos about MMM method, I published it a long time ago…
Other readings :
MMM howto in video : https://www.youtube.com/watch?v=6RiuwqzjqlQ
https://www.erinsaudiocorner.com/loudspeakers/ interesting because for loudspeaker tests, MMM (called Moving Mic Average by Erin) is compared to Klippel’s results : it simply proves that MMM is very comparable to Harman’s In-Room prediction based on anechoic measurements.