Wet means the two primary reeds are tuned farther apart in pitch. When you add the two, the ear hears the resultant pitch as an addition of the two sine waves. The difference in the pitch gives you a slight higher pitch signal that rides on the main note. This gives an effect known as tremolo, or almost a vibrato tone. It really is very pleasing to the ear IMHO. It does not seem to be popular among tradtional Cajun accordionists however.
I am working with my first LA handmade which is very dry. I started out with an Ariette which was very wet in comparison. While a vastly inferior instrument, the Ariette does have a very pleasing sound.