Here, one-part of the LM393 (IC1) works as the voltage comparator, a reference voltage at the inverting input (pin 2 of IC1) is set by the multi-turn potentiometer (P1). When the analog value, inputted to the non-inverting input (pin 3 of IC1) from the sensor pad goes beyond this pre-setted value, IC1 will output a digital value to indicate that the setup threshold value is reached, and the sensor is activated.
Note that, this steam sensor circuit offers two outputs; Digital (DO) and Analog (AO). If you only want a logic-low (L) level output when a threshold is reached, just use the Digital (DO) output. But if you are looking for a varying voltage value as the output to indicate the sensing parameters, the Analog (AO) output is your best option. The green-LED (LED1) works as the power-on indicator, and the red-LED (LED2) indicates active-state of the sensor circuit.