Many accordions have more than one reed sounding per note in the middle register, and how those reeds are tuned relative to each other defines what's called "wet" or "dry" tuning.
How it works
If two reeds tuned to exactly the same pitch sound together, there's no beating and the tone is "dry" - clean and pure, closer to a single-reed sound. If the reeds are tuned slightly apart from each other in frequency, the tiny difference creates an audible tremolo (rhythmic "beating") as the two sound waves drift in and out of phase - the wider the tuning gap, the faster and more pronounced ("wetter") that tremolo becomes.
Musette
"Musette" is the common name for a pronounced wet tuning, historically associated above all with French musette and various folk/traditional playing styles; "true musette" traditionally refers to a three-reed (MMM) middle register tuned this way.
Why it matters when buying
Wet/dry tuning is a deliberate, largely permanent tonal characteristic of a given instrument (changing it requires a technician retuning reed sets) rather than a switchable feature on most accordions, so it's worth listening for specifically - a beautifully maintained accordion tuned "wetter" than you want will still not sound like what you're after, and vice versa.
