That looks like a car seat occupancy sensor for airbag/seatbelt-light activation. It is a simple switch. If one or more of the circles has enough weight on it to close the contact, the switch is closed - that is, the two wires on the connector are shorted together. Otherwise, the switch is normally open.
To use it, connect one of the wires to ground (it doesn't matter which one), and the other wire to an Arduino pin. Assuming you have an Uno, write the following code to read it:
const byte switchPin = 2; // connect the switch to pin 2, the other side to ground
const byte ledPin = 13; // use the built-in LED as an indicator
void setup() {
pinMode(switchPin, INPUT_PULLUP);
pinMode(ledPin, OUTPUT);
}
void loop() {
digitalWrite(ledPin, !digitalRead(switchPin));
}
You can find these switches (they come in a variety of shapes, and are about 6-10 inches across) affixed to the underside of the passenger seat in cars made in the last 5 years. It takes more than 50 pounds on the seat to close the contacts when installed.