what the heck is a ketone

It is a new way to make people lose weight... from their wallet.

I assume they are talking about ketone bodies. When I googled this "supplement," I found one that contained beta-hydroxybutyrate, which is a ketone body. The marketing abbreviates this as BHB.

A ketone body is a molecule produced by the body when fatty acids are metabolized. Fatty acid metabolism happens when liver glycogen stores are depleted, which occurs during fasting, carb restriction, and other situations that probably aren't relevant for this discussion. Ketone bodies are responsible for the odor one's breath may have when they are in ketosis or ketoacidosis (note that these are not the same thing; the second one can be life threatening). Some, but not all, organs can use ketone bodies for fuel, if glucose is not available.

I cannot imagine why anyone would want to consume ketone bodies. If one is following a carb restricted diet, then the body will make its own ketone bodies as part of the normal metabolic process. They are a result of being in ketosis; they do not cause it. While they are an energy source in some cases, they are not a metabolically preferred energy source for the organs that can use them.

If the level of ketone bodies in the blood exceeds what is consumed, then they're excreted in the urine. In other words, this is yet another "supplement" that is entirely or almost entirely peed out.