In simple terms it's when you trick your body into using your own BODYFAT as its main energy source instead of carbohydrates. The Keto diet involves going long spells on exceptionally low (no higher than 30gm per day) to almost zero gm per day of carbs and increasing your fats to a really high level (to the point where they may make up as much as 65% of your daily macronutrients intake.) The idea behind this is to get your body into a state of ketosis. In this state of ketosis the body is supposed to be more inclined to use fat for energy- and research says it does just this. Depleting your carbohydrate/glycogen liver stores and then moving onto fat for fuel means you should end up being shredded. The keto diet is very popular method of losing fat quickly and efficiently.