The starter motor uses more current than most of your car's wiring or switches will handle. To alleviate this problem, the solenoid (an electromagenetic coil) is energized by voltage from the ignition switch. The solenoid then moves a large piece of copper across two terminals which route current directly from the battery to the starter motor.
The fast clicking noise is caused when the solenoid is energized, current flows from the battery to the starter, and then battery voltage drops so low the solenoid loses power. There is a spring in the solenoid which moves the copper bar away from the terminals when the solenoid loses power. As the drain on the battery drops, the solenoid gets power again, and the cycle repeats rapidly, causing the clicking noise.
The fast clicking noise is caused when the solenoid is energized, current flows from the battery to the starter, and then battery voltage drops so low the solenoid loses power. There is a spring in the solenoid which moves the copper bar away from the terminals when the solenoid loses power. As the drain on the battery drops, the solenoid gets power again, and the cycle repeats rapidly, causing the clicking noise.