Like any other battery, your car battery leaks power over time. If you do not drive your vehicle, your battery will eventually lose the capability to supply the necessary voltage to start your engine. The battery does not need to be completely exhausted of voltage to be considered a failing volt storage unit.
When you use your voltmeter to test your battery, you get a display of the amount of voltage stored inside. Without the engine running, a fully-charged battery registers 12.6 volts.