Moral Platonism holds that there is an object that exists that is good. As in, we have things like rocks, trees, mountains, and a good. So not something that is described as a good thing (like a cake is good), but a thing that is good ("look at that good roaming the countryside").
I think for most people, the initial reaction to this concept is that it's nonsense. How could it be that good is a thing? It makes no sense. Dr. William Lane Craig calls it unintelligible.
I'd agree that it seems completely crazy on the face of it, but my obligation to be as unbiased as possible reminds me that plenty of scientific discoveries have seemed to fly in the face of common sense. So even if this idea of a good thing seems to be nonsense, it doesn't mean that it actually is. We actually have to find out if it's nonsense. Is a good thing simply difficult to understand, or is it actually unintelligible like a square circle?