The rain sensor detects water that completes the circuits on its sensor boards' printed leads. The sensor board acts as a variable resistor that will change from 100k ohms when wet to 2M ohms when dry. In short, the wetter the board the more current that will be conducted.  

Pins:

A0.......... Analog output

D0......... Digital output

GND..... Ground

VCC...... Positive voltage (input: 5v for analog 3.3v for Digital.)

The other two pins on the comparator board should be connected to the rain sensor board. 
​The kit can be used for weather monitoring purpose also.

The output is in both digital and analog forms: 

In digital: The output is either 0 or 1 and in Analog you get varied output. 
Its output current is 100mA which means that it can easily trigger a relay, a buzzer and an LED. 
The sensitivity can be adjusted with the potentiometer on the board. 
LED lights up when there is no rain and output(D0) is high and when there is raindrop the output is LOW.