I believe this device is what is often referred to as an Arduino Rain Sensor.
It works like a light detector, or potentiometer - the resistance goes down as the plate gets wet. To use it, I would connect it as one leg of a voltage divider, or if you want to get fancy, as an input to an op-amp in comparator mode.
@ger_mcmahon, to make this in it's simplest form, the voltage divider, you will need a proto bit and a resistor, say 10K ohm. Connect the resistor between the ground and signal on the output side. Then wire the output from the rain sensor between the signal and the +5 volt line, also on the output side. The voltage on the signal line on the output from the proto bit will vary. If you analogRead the input pin the rain sensor-proto bit is attached to, the reading will be low (less than 100) when dry, and high (close to 1000) when wet. Merely damp will be in-between.