Thin, metal, circular springy rings that fit on the ring landing area on a piston. One of their purposes is to maintain cylinder pressure by not letting the gasses that cause combustion leave the combustion chamber. By creating this seal, the combustion forces the piston down, driving the crank. Another purpose of the piston rings is to maintain the separation between the combustion chamber and the engine oil. The rings will scrape the oil from the cylinder walls to ensure the oil isn't getting burned in the combustion process.