The phrase "burning fat for fuel" refers to a metabolic state in which the body uses dietary fats and stored triglycerides as its primary energy source. This process is called lipolysis, where enzymes break down lipids (fats) into glycerol and free fatty acids, which can then be used to produce ATP through beta-oxidation in the mitochondria. During fasting or prolonged exercise, when glucose levels are low, the body turns to fats as an alternative energy source for muscle contraction and other vital functions. This is a more efficient way of generating energy compared to carbohydrate metabolism, as it produces fewer waste products and generates higher amounts of ATP per molecule of fatty acid.