As above the compression ratio is the how much the charge in the cylinder is compressed by the movement of the piston before it is ignited by the spark.
The more you can compress the gas the more torque you can get back out of the engine when it's ignited, however as the compression in the cylinder is governed by the gas law the temp will go up as it's compressed and if you compress it too much it's self ignite like a deisel engine.
If you lower the compression of an NA engine you'll loose power, the comperssion needs to be lower on a SC as the gass entering the cylinder is already compressed, but you get more power as you can cool the gas between the 2 compression stages.
Although roots compressors are not very efficent meaning more of the work done on the gas is turned into heat so the comp ratio has to be reduced considerably compared using a centrifugal compressor