High Compression refers to the compression ratio of the pistons.
Basically the higher the ratio, the more they compress the air/fuel mixture, and the more power they make for their displacement.
You don't really get much of a choice but basically a high-compression engine means that it's a high performance engine, which makes a lot of power for its size. For example, the 2006+ civics. the regular ones *DX, LX, EX* have a regular compression 1.8 liter engine that makes 140 hp. The Si which is a higher-compression engine has 2.0 liters but makes 197 hp. It makes a lot more power per liter because it's higher-compression.
The only downside is that because it compresses the air/fuel mixture more, it heats it up alot more too, which can lead to pre-detonation, basically it explodes for it's supposed to, and can be heard as a pinging sound. This is also why they all need premium gasoline. Premium gasoline resists detonating, so it would be safer to have a higher-compression. *for civics, the DX, LX, and EX use regular gas while the Si needs premium*
hope that explains it
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