The Raindrop Sensor uses the Conductivity to determine whether the rain drops is on the board or not. When there are no liquid on the board, the reading will be very small (0 if no liquid), however even one drop of rain, can increase the conductivity very much and make the reading to extremely high (100).
 
Working Principle
The RainDrop Sensor use the conductivity between the two electrodes to determine the change of the water drop on the plane. When there is no water drop on the plane, the material between the two electrodes is air, result in very low conductivity. However, when the water drop is on the plane, the conductivity between the electrodes will become very high since water is a good conductor.