What does it mean if there is rust on spark plug?

Rust on a spark plug typically indicates that moisture has infiltrated the combustion chamber, leading to corrosion. This can result from water entering the engine due to improper sealing, a blown head gasket, or excessive humidity. Rust can impair the spark plug's functionality, causing misfires, poor engine performance, and difficulty starting. It’s advisable to inspect the engine for underlying issues and replace the affected spark plug.