A Calorie (Caloric) Deficit is when you consume less calories in a day then what your body needs. This causes your body to use the fat storage in your body. This is what makes you lose weight and get shredded. The number of number your body consumes a day depends on many factors for example height and age play a big role in how many calories you consume a day. The average person consume roughly 2000 calories a day if you go on a caloric deficit diet you would be consuming 1700-1500 depending on how much you want to lose per week.