try read in this link for beginners :
https://www.dietdoctor.com/low-carb/keto
there are two scenarios where we have Ketones in our body in higher amounts
1.
is the almost poisonous way where type 1 usually has so high blood glucose that their body is poisoned by it as they do not have insuline: so they cant use their glucose in their body, then their body that always need energy start using the eaten protein AND muscle tissue in the serverely diabetic body and fat and transform it into ketones that is another kind of energy substance your body can use when not having access to glucose as an energy source.
2. when we deliberately go so low in carb intake that our body starts using the proteins and fat we eat as energy by changing it into ketones where we instead of glucose use ketones , but where our body is not poisoned by high levels of glucose like in the first mentioned examble...
so you see there is both a terribly poisoning condition where the body uses Ketones and the other where the glucose"starving" body changes to making ketones as a kind of alternate energy source.
it is actualy a natural form of energy transformation in humans , but in modern times people usually never starve long enough to scwitch to fatburning as a main resource of energy, we hardly ever starve even for a few hours nowadays
http://antranik.org/the-catabolism-of-fats-and-proteins-for-energy/
If I am wrong in this explanation , then somebody please correct me