Red light means the probe on the temperature controller is not reached. Blue light means it is at the set temperature. When the red light is on power is supplied to whatever heat source is plugged into the temperature controller. When the desire temperature is reached (whatever temperature you set your controller to) the controller will shut off to prevent overheating and the red light will go off. When the temperature drops the controller will turn back on the heat source plugged into it and the red light will go on.