1 MET = the energy (oxygen) used by the body as you sit quietly, perhaps while talking on the phone or reading a book. The harder your body works during the activity, the higher the MET.

• Any activity that burns 3 to 6 METs is considered moderate-intensity physical activity.
• Any activity that burns > 6 METs is considered vigorous-intensity physical activity.

More information in CDC site
In another expression, a MET also is defined as oxygen uptake in ml/kg/min with one MET equal to the oxygen cost of sitting quietly, equivalent to 3.5 ml/kg/min.
By using METs, kcal/min is calculated as follows: METs × body weight in kilograms ÷ 60
Oxygen consumption in liters per min= METs × body weight in kilograms × 3.5 ÷ 1000