Since your car is a hybrid, the engine stops every once in a while and the car runs on battery. When engine stops, there is practically no noise to warn pedestrians in slow driving. This feature adds an artificial engine sound to the car linked with movement. If it is on and the car is moving at slow speed on battery, it will make a sound to warn pedestrians.
So the light just denotes the system is off. You could try pressing the button for a few seconds to turn it back on, but if that doesn't work there may be some issue with the system.
