Flame sensor is only sensitive to the light in the frequency band of infrared and ultrviolet.
This breakout board has 3 pins: sensor output signal, power, and ground. The sensor output signal is an analog signal. In an indoor enviroment, the signal output is around 0V. When there is flame or other infrared source, the voltage of output signal will increase. When the intensity of light is high, it can output as large as 3V.