If light is on, overdrive mode is switched off in the transmission. If you switch overdrive back on and the light remains on, it’s a transmission or electrical problem. It’s safe to drive your car to a service shop.
Usual problems: failed overdrive-off switch, electrical problem, defective transmission module
Overdrive is a device that allows an extra high overall gear ratio for high speed cruising with less torque and less engine power. Overdrive drops the car’s engine, reducing wear and saving fuel. Overdrive-off mode gives you the option of moving slower with greater engine power.