Transponder keys provide an extra level of security to your car. They contain a very small computer chip inside them which is used to authenticate the original car key and the duplicated car key. When you turn the key in ignition, the Engine Control Unit (ECU) will send a coded message to the key.
The key will receive the coded message and will respond back to the Engine Control Unit. Only once when the messages are authenticated by the car, your car will start.