A piston ring is a metal ring that fits into a groove on the outer surface of a piston in an engine. Its primary function is to create a seal between the piston and the cylinder wall, preventing the escape of combustion gases and maintaining compression. This helps improve engine efficiency and performance. Piston rings also help regulate oil consumption by scraping excess oil off the cylinder walls and returning it to the oil sump. They are essential components in various types of engines, including those found in cars, motorcycles, and other machinery, ensuring smooth operation and longevity.