Soil humidity sensor is used to measure the humidity in the soil or to measure the liquid level in a small scale. Humidity sensor probs must be stuck in the soil in operation. The resistance of the liquid or the soil will generate a potential difference between two probs. It is possible to measure the humidity respect the magnitude of this potential difference. The conductivitiy of the material increases as the humidity of the soil increases. It is possible to set a precision level via the trimpot placed on the PCB. It is possbile to use this sensor with Arduino and many other microcontroller platforms.
You can read analog data from A0 pin and Digital data that you adjusted via the potentiometer from D0 pin.