What are MMO songs?

Factual Questions
1

Or, in a concert context, what does “MMO” mean if it is written that the artiste will sing MMO songs?

2

Are you referring to “Music Minus One” pieces? These are recordings of the accompaniment; you provide the solo part (instrumental or vocal).

3

Thanks. Sounds like it should be called Music With One (rather than Minus One), but there we are.