In my opinion it means the snake is feeding/shedding/pooping regularly and in overall good health. Also it also usually means there at least 200grams. Ive seen ads for "well started males" and theyre over 400 grams already. Thats basically breeding size lol
It's kind of a broad spectrum. My take on it is it's usually a animal that just hasn't sold. It doesn't mean there's something wrong with it, just hasn't sold. Ask whoever the seller is for the details on the specific animal your looking for.
I'd say that they are sometimes still "hatchlings," but not in the sense that they're tiny and frail. They have taken multiple meals and shed. I'm sure there have been juvenile snakes labeled as "well-started." The term's concrete meaning varies from breeder to breeder, but generally is just a heads-up that says "hey, this snake is growing and healthy" to a potential buyer.