Ben_H:
As best as I understand it, as another relative newbie, is that... Accordions have three distinct reed 'lengths', a bit like church organs with the 4', 8' and 16' ranks of pipes. On an accordion the Lowest set is referred to as 'L', and called Bassoon on a switch. This plays an octave below the Middle set, which is called 'M' and referred to as 'Clarinet', while the Highest 'H' set is an octave above and is called 'Piccolo'. For musical reference, the normal Middle or M set plays a Middle C note when the C key nearest the top of the accordion is pressed.
What muddies the waters though is that many accordions will have more than one set of Middle or 'M' reeds. If they were tuned identically then of course all that would happen is you get a stronger sound when you select the switch that uses more than one. But to get the typical French, often called 'Musette' sound, there must be the normal 'M' set of reeds tuned to standard pitch, then another set of 'M' reeds tuned fractionally lower, and yet another 'M' set tuned fractionally higher, which gives a distinct beat to the sound and produces that typical rich wavering sound you hear on French café accordion music.
So an accordion that is described as LMM would have one Low set of reeds and two Middle sets of reeds. There is no way of knowing what the actual tuning of each set of 'M' reeds is without hearing the accordion, or the seller telling you. It is possible to get a very plausible musette sound from just the two 'MM' sets, but to get a true musette sound, you need an accordion that has 'MMM'. Some might just have that, some (4 voice models) may have LMMM or MMMH even.
An accordion that is 'LMH' will not be able to produce a musette sound so would be unsuitable for those wishing to make that sound, but would/could be ok for many other uses such as classical or folk music.
Most players of folk, ceilidh, musette and other popular styles would not wish or need to have an 'H' reed set so would typically buy an accordion that is 'LMM' or 'MMM'. The 'H' Piccolo set is normally used by eminent classical performers to very good effect though, so it depends on what musical tastes you have.