A calorie is a unit of energy. Weight loss is typically achieved when total calories expended (used) exceed total calories consumed, resulting in a caloric deficit. For example, during a day if you consume 1,200 calories and use up 1,700 calories you will be in a caloric deficit of 500 calories. Clearly there are two parts of this weight loss equation which you can alter – calories consumed (through diet) and calorie expenditure (which is usually achieved through exercise and lifestyle but can also be aided with weight loss supplements).
Successful weight loss workouts will aim to tilt the calorie balance towards a deficit by using up a large number of calories during the workout, as well as increasing caloric burning for many hours after and heightening the overall metabolic rate through enhanced body composition.