Complex sentence: When the ignition switch is turned on (when the key is turned to start the car), the small electrical current tells the starter solenoid to relay the large electrical current to the starter motor.
Split into simpler sentences: When the ignition switch is turned on (when the key is turned to the start position), a small electrical current is sent to the starter solenoid. This causes the starter solenoid to relay a large electrical current to the starter motor.