Plain English Meaning
When the cylinders inside the engine get hotter than they're supposed to, usually because the engine isn't getting enough cooling air or because the fuel mixture is too lean. If it keeps happening, parts of the engine can be damaged.
Full Definition
A condition in which the temperature of one or more engine cylinders rises above the manufacturer's recommended operating limits, typically caused by prolonged ground operation, extended climbs at low airspeed, lean fuel mixtures, or inadequate cooling airflow. Sustained cylinder overheating can lead to detonation, preignition, loss of power, and progressive engine damage including warped valves, scored cylinder walls, and cracked cylinder heads.