A Standard American Diet is high in carbohydrates. The body uses those carbohydrates, glucose, and sugar for energy. When you start a nutrition strategy such as Keto that is low in carbohydrates and high in fat, the body has to adjust its preferred fuel source from carbohydrates, glucose, and sugar to fat. The fat energy will come partially from your dietary intake, but mostly from your current fat stores provided you are eating the appropriate amount of dietary fat.
The objective of Keto is to get into a metabolic state called ketosis. Ketosis is the process by which the body is fueled by stored fat. Ketones are the byproduct of the breakdown of fat molecules. With time, your body will become fat adapted (burning fat for fuel instead of carbohydrates, glucose, and sugar), hunger levels will radically decline, inches will drop off your body, energy levels soar, mental clarity skyrockets, and so much more.