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When you turn on your gas stove (or gas turbine if you are in industry) each morning and you set the knob to "5", you would like to get the same amount of heat from the burner from one day to the next, even though the composition of the natural gas feeding your appliance may vary from day to day. If the Wobbe index of the gas remains constant from day to day then the heat output of your appliance will remain the same from day to day. The Wobbe index is a measure of the heat content of the gas that flows through an orifice of constant size under constant pressure per unit of time.
Based on elementary physics, Goffredo Wobbe in 1927 deduced that said heat content is a) proportional to the heat content of a cubic foot (or meter) of gas at standard temperature and pressure (STP), and b) inversely proportional to the square root of the density (or specific gravity) of the gas. I.e., at STP, lighter gas flows through the orifice