Just dealt with this today.
Overheat is a single flash of the light, repeated...
Hit the stop button.
3 flashes followed by 3 flashes (3 3 code) = engine overheated, check coolant level, add coolant. Radiator pressure cap is under the cover right above the on off switch on the genny, remove one screw to pull the cover and get in there.
3 flashes followed by 4 flashes (3 4 code) indicates a problem with the inverter board overheating. I'd think this could be serious, but have no experience with it.