Output voltage might refer to a number of different components in a car stereo system but it is most likely to refer to the output voltage of the stereo system's amplifier.
An amplifier, as its name suggests, amplifies a signal that is fed into its input. The amount that an input voltage is amplified depends on the size, type and quality of amplifier and its setting. For example, an input signal voltage of .5 in an amplifier set to amplify by a factor of 20 would result in a output voltage of 10 volts. It is important to note that amplifiers can only amplify a signal up to a given capacity (or "rail"), and exceeding this capacity only leads to harsh distortion.
Speaker output, however, is measured in watts rather than voltage. This is because the amount of impedance that affects the signal transmitted to the speakers needs to be taken into account before a final output can be determined.
Output voltage might also refer to other components in a car stereo as well. It may refer to the head unit (CD player, radio or other similar component), in which case it refers to the output voltage of the signal emitted by the component. It may also refer to the output voltage of a signal processing unit such as an equalizer or crossover. Like an amplifier, these units have their own voltage tolerances.
