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.
A worked example
A 70 kg person jogging at about 7 METs for 30 minutes burns roughly 245 calories: 7 × 70 × 0.5 hours. The same half hour of brisk walking (about 3.5 METs) burns half that — around 123 — while 30 minutes of vigorous running (about 9.8 METs) burns closer to 343.
Body weight scales the result directly, so a 90 kg person doing the identical jog would burn about 315 calories instead of 245. That's why the same workout “counts” for more the more you weigh.
Why it's only an estimate
MET values are population averages for a typical effort level, but two people doing the “same” activity can differ a lot. Fitness, intensity, terrain, and technique all move the number, and the simple calculation doesn't subtract the calories you'd have burned at rest anyway.
Treat the figure as a reasonable way to compare activities and rough out a session, not a precise ledger to “eat back.” Fitness trackers refine this with heart rate, but even those are estimates.