Windshield Rain Sensor
Rain sensors come as a feature on some vehicles. It is a device attached to the windshield that detects the presence of water on your glass; this allows your vehicle to turn on and adjust your wipers automatically in different types of weather.
Picture sourced from Mitsubishi Motors at http://www.mitsubishi-motors.com/en/spirit/technology/library/sensor.html
Having the rain sensor feature changes what windshield will be needed during a replacement. The difference between a rain sensor windshield and a standard windshield is that there is a small area (usually behind the rear view mirror) that is blackened out to hid the device. The dark area has a small view port with no tint or shading that allows the sensor to operate.
By L.Kenzel - Own work, CC BY-SA 3.0,https://commons.wikimedia.org/w/index.php?curid=7014340
How Does it Work?
Most commonly a rain sensor consists of an infrared light set at a 45 degree angle to a clear portion of the glass. The “sensor” is a photodiode sits across from the infrared LED that measures the extent that light has been reflected versus refracted. As water rests on the glass, it reduces the amount of light reflected and the sensor identifies the difference. That is what allows your vehicle to know if there is water on the glass and how much.
By Puppenbenutzer - Own work, CC BY 3.0,https://commons.wikimedia.org/w/index.php?curid=6505218