People with a high metabolism burn more calories than people with a low metabolism. Intense exercise requires more energy than resting on the couch. You burn more calories digesting good food then when you are downing empty calories in junk.
The goal is to have the calories you eat match the calories you burn or it gets stored as fat. Knowing your threshold can help you figure out how many calories you need to keep your energy needs in balance and your body at an ideal weight.