This is not a precise term, you cannot go and buy "moist" and "dry" vanilla beans. But the pods can be prepared to different degrees of dryness, similar to prunes. And the ones with more remaining moisture have more taste and are generally higher quality. This is why they specify it. If you only have access to dry pods, I will still try out the recipe with them, just change the source for the next buy.
Regarding the comment above, I have never had vanilla beans with a grade denoted on the packaging. But I have had both moist and dry ones, so it isn't a case of only one grade being marketed to end users. It is surely better if you can distinguish them based on some kind of grading, but chances are, you won't know what grade you are buying.