A rain sensor usually works the same as a soil moisture sensor in which is determines the resistance between two points. Typically a rain sensor is two traces running close but never touching on a circuit board. The idea is if moisture (rain) runs across the traces it will reduce the resistance and give you an idea of present moisture.
The rain sensor will give you a reading of 0 - 1024. Not all sensors will have the same sensitivity and you should do some tests by dripping different amounts of water on the sensor to simulate different rain conditions.
[Integer] 0 - 1024
Here are some readings from my actual setup I used to help calibrate my readings. Use them as a reference to get started but I suggest do your own tests for ideal results.