Different activities burn energy at different rates. The MET (metabolic equivalent) system captures that, letting you estimate the calories an activity burns from just your weight and how long you do it.
How MET works
One MET is roughly the energy you use at rest — about one calorie per kilogram of body weight per hour. An activity's MET value scales that up: a 7-MET jog burns about seven times your resting rate. Multiply MET by your weight in kilograms and the time in hours for an estimate.
These are averages; real burn varies with intensity, fitness, and body composition. Use the number as a guide, not an exact measurement.