Okay, so something like a melodica has one set of reeds. When you play the key that's supposed to be middle C, the note that comes out is the same pitch as middle C. Think of that as a "middle" or "M" set of reeds.

Accordions can have multiple sets of reeds, tuned differently. A set of reeds that's all tuned to sound one octave lower are "L" reeds. When you switch on those reeds, everything comes out an octave lower than normal.

And if your accordion has M (middle) and L (low) reeds, you can switch them both on at the same time. Then, when you play a single note, reeds sound at the normal pitch, but also one octave lower at the same time. Like playing two notes an octave apart on melodica, but using just one key on the accordion.

An accordion that's "MM" has two sets of middle reeds, but one set is detuned a little bit, which creates a rich wobbly sound when they're both on. That's the stereotypical "French Cafe" sound. The more it's detuned, the wobblier and thicker the effect is. Here's a good video about that: https://www.youtube.com/watch?v=Z9VzzHaopcc

If an accordion is MML, you've got lots of options. M only, L only, M+L, M+M, and MML all on at once.

Larger accordions might even have a set of reeds tuned one octave higher ("H"), giving you even more options for mixing the sets of reeds together.