Compression is squeezing something so that it occupies a smaller volume. Ratio is the proportion between two quantities or numerical values. The basic unit of an engine assembly is a combustion chamber and piston combination. This ratio is a proportion which denotes the degree of compression achieved, in an internal or external combustion engine, when the driving pistons compress the air-fuel mixture in every engine stroke. It is a ratio of air-fuel mixture volume, when a piston is at the end of its up-stroke, to the air-fuel mixture volume at the end of its down stroke, in a combustion chamber. It is a measure of the air-fuel mixture compression and density achieved, in every engine stroke.
The combustion chamber is a cylinder, which is fed with a mixture of air and vaporized combustion fuel. A piston is placed inside the combustion chamber and it makes up and down movements, according to pressure applied. The air-fuel mixture, fed inside the combustion chamber, is ignited, when you turn on the ignition of the car.
The explosive combustion makes the piston move up and every time it comes down, it compresses the air-fuel mixture again, igniting it in the process.