An engine's compression ratio :
ratio of the volume of gas in the cylinder when the piston is at the top of its stroke to the volume of gas when the piston's at the bottom of its stroke .
It shows how well the air fuel mixture is compressed into combustion chamber.
More compression ratio ,more efficiently it burns and the more power is made.
High power and performance ,outputs more heat and may have noise and vibrations at cases.If you require high power we should have good compression ratio.