A keto diet is a low-carb diet that results in ketosis. To achieve this, the diet has to be very low in carbohydrates and high in dietary fat, with a moderate amount of proteins.
This diet involves depleting your body of carbs and forcing it to break down fat for energy.
Ketosis is a normal metabolic process, something your body does to keep working. When it doesn't have enough carbohydrates from food for your cells to burn for energy, it burns fat instead. As part of this process, it makes ketones.
It will take a few days or weeks to feel your absolute best on keto. It is possible that you may experience symptoms of carbohydrate withdrawal at first, but once you become fat-adapted, you'll find that you won't crave carbs as much anymore.