Ah, I know what they're doing! I've seen the Japanese instructions for this. Those must be Ume - which are somehow always translated as plums, but are actually closer to being apricots? The first set of jars you put the green ume in layers with salt. In Japan they also add some red Shiso leaves or such to dye the salted plums red. Those end up as the ubiquitous salt pickled plums you see in classic Bento rice as Umeboshi.
The second set of jars are layered with rock sugar and a hard liqour (soju/shochu) is poured over. The plum wine is ready by the next year's ume harvest.
I've always wanted to try this, but can't get my hands on Ume plums. I never hear of uses for the ripe plums, vaguely remember hearing they weren't so good?
has a lot of quick info on that type of plum.
Now I'm going to have to look into this again with an eye to using whatever I have on hand. Fruit liquor and salt pickles!
-edit to add-
HAD to know, so searched for making umeboshi with regular plums and found this
, so now I have a new project!