From SKYbrary Wiki
A supercharger is a mechanically driven, forced induction device that is utilised in a piston engine to enable it to produce more power. Its purpose and function is similar to a turbocharger with the principal difference being that it is mechanically driven by a direct connection to the engine whereas a turbocharger is turbine driven using engine exhaust gas flow.
In a normally aspirated piston engine, intake gases are drawn into the cylinder by the reduced pressure created by the downward stoke of the piston. The mass of the air that is drawn into the cylinder, in part, limits the power production of the engine, A supercharger is an engine driven mechanical device which powers a compressor. This compressor draws in ambient air, compresses it and then feeds it into the engine intake resulting in a greater mass of air and, proportionally a greater amount of fuel, entering the cylinders on the intake stroke. The additional air and fuel result in a significant increase in the power production of the engine compared to a normally aspirated version of the same engine. Although it takes a significant amount of engine power to drive the supercharger, the power increase due to the effect of the supercharger more than compensates for that loss.