Ketogenic Diet is a very low carb, high fat diet. The idea of this diet is to eat less carbs and replace them with fat and protein, resulting in a state called ketosis.
Ketosis is a process that happens when our body doesn't have enough carbohydrates to burn energy, producing ketones which will become our body and brain main source of energy. This could be useful in epilepsy when children with seizures do not respond to medications.
Nowadays, it is related to weight loss. This is because the first thing we lose when we do not eat carbs is glycogen and water, resulting in immediate weight loss.
NEGATIVE ENERGY BALANCE
The real reason why it can result in fat burning is calorie deficit or negative energy balance. Ketogenic Diet is another way to get a negative energy balance, but is not the only way to consider fat burning.
On the other hand, you have to consider that 1 gram of fat equals 9 calories vs 1 gram of carbs or protein equals 4 calories: this is the reason why we need to control intake in order to be at a negative energy balance.
Keto diet could be useful but is not for everyone. We should consider that limiting one macronutrient or a food group could take the person to some adverse effect like anxiety or carb binge.