The "check engine light" is a warning indicator on a vehicle's dashboard that signals a potential issue with the engine or its components. It can illuminate for various reasons, ranging from minor issues like a loose gas cap to more serious problems such as engine misfires or emissions system failures. When the light turns on, it's important to have the vehicle diagnosed using a OBD-II scanner, which reads error codes stored in the engine control unit. Addressing the underlying issue promptly can help prevent further damage and ensure the vehicle operates efficiently.