Paul, the MM/MC switch selects whether the pick-up cartridge you are plugging in to the PHONO IN RCA sockets is a Moving Magnet (MM) or Moving Coil (MC), literally referring to the internal mechanism inside the phono cartridge, and thus its output voltage.

AudioTechnica have a good article explaining the difference here:
https://blog.audio-technica.com/audio-solutions-question-week-differences-moving-magnet-moving-coil-phono-cartridges/

MC cartridges typically cost several times the cost of an MM cartridge and are considered "audiophile" equipment, but have a much lower output than MMs and hence need much higher gain at the pre-amp end: in this case, the Magix interface. That's why the MM/MC switch exists: you'll find a similar switch or setting on audiophile pre-amps or amplifiers.

The vast majority of phono cartridges sold are MM and, if for some reason you can't find out online which type your phono cartridge is, use the MM setting first. If you get no output, or very faint or tinny output, try the MC setting but be sure to lower the volume of any monitoring equipment first and definitely remove headphones! That way, you won't damage loudspeakers (or your ears!).

Finally, in case it isn't clear, the Magix is expecting to have your phono cartridge/turntable plugged directly into its PHONO IN sockets, just as you would connect it to a DJ mixer or audiophile amp or pre-amp. It's expecting cartridge-level input, not line level.

I hope that helps. Sometimes it's good to be old, and remember this stuff from the 1970s!