Plain English Meaning

The fuel mixture in the cylinder lights itself early, before the spark plug fires, because something inside the cylinder is glowing hot. This forces the engine to push against burning gas at the wrong moment and can quickly wreck parts inside the engine.

Full Definition

Preignition is the ignition of the fuel/air mixture in a cylinder before the spark plug fires, caused by a hot spot inside the combustion chamber such as a glowing carbon deposit, an overheated spark plug electrode, or a damaged exhaust valve. Because combustion begins too early, the rising piston fights against the expanding gases, producing severe cylinder pressures and temperatures that can damage , valves, and cylinder heads.