It'* the ratio of the volume of air and fuel before and after the compression stroke. Easy exagerated example: a cylinder has a volume of 8.5 liters at BTC (bottom dead center, the very instant before the piston begins to move upwards), when it has reached TDC (top dead center) the mixture has been compressed to 1 liter.
With a forced induction engine, 8.5 is the general compression ratio used. If it is higher, the fuel will ignite on its own due to pressure, much like a diesel engine. NA enginescan use 10, 11, 12, higher numbers to get more power because the pressure is lower than a blown engine.