MET is an abbreviation for Metabolic Equivalent is a term used by physiologists to describe the energy cost of doing physical activity, in terms of the amount of oxygen that was required to complete the activity.
The way it works is that we have determined that at rest, we take up and use 3.5ml/kg/min of oxygen, which was assigned the value of 1 MET.
We know that the higher an activities intensity level, the greater the oxygen requirement necessary to produce energy. For example the energy cost of walking is less than the energy cost of sprinting.
Over the year the metabolic cost of almost every physical activity have been tested and the 3 tables below are a good illustration of how activity intensity, energy cost interact (from whyiexercise.com).