A voltmeter is a device that measures the voltage in an electric circuit. A dash mounted voltmeter can be used to monitor the voltage and battery condition of a car.
The VIN is the Vehicle Identification Number. This number is unique for every vehicle and allows for tracking the history of that vehicle. This is useful when buying a new car and knowing what has happened to the vehicle in the past, where is was made (for plant specific recall purposes), insurance, and many other vehicle specific needs.
Valve timing is the order in which the exhaust and inlet valves open and close with respect to the piston movement and firing order. Valve timing is controlled by the camshaft.