Compression Height is a measure of the distance between the center of the wrist pin and the top of the piston head. There's alot to fit between these two points.

Working from top to bottom, we find two compression rings which are designed to keep the air in the cylinder, followed by an oil ring at the very bottom. Sometimes called a scraper ring. It's job is to push the oil that builds up on the cylinder wall down into the oil sump, away from the combustion chamber.

Most race engines, or engines rebuilt with stroker motor applications in mind will modify the compression height in order to fit a larger stroke in the engine.