The voltmeter is an instrument which measures voltages (tensions or differences of potential) between both ends of an element in a circuit through which there is an electric current circulating.
It is based on the same principle as the ammeter and it is connected in parallel with the element in order to measure the difference of potential between its two ends.
Observe how the voltmeter is connected in the visual. This voltmeter measures the voltage between the two terminals of the light bulb. The voltage changes when there is a variation of the intensity going through the light bulb.