That is the gas ignition light for your water heater. With the water heater switch on the electric flame ignition will try to light the gas. If no gas is available or there is some other failure of the igniter circuits the light will remain on. If it lights ok the light will go off.
After 10 attempts the system will LOCK OUT further attempts so you then must go out and reset the system using the rubber covered reset button.
If you are heating with AC, (rocker switch outside) the gas switch inside can be off. It can also be on to give a faster recycle time for several showers since the gas heats water faster than electric.
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