A voltmeter measures the potential difference, also called the voltage, across a component. Potential difference is measured in volts, symbol V. The voltmeter is connected in parallel with the component it is measuring.
To do this, you join the two terminals of the voltmeter to the two ends of the component, so the voltmeter sits on its own branch that goes across the component. The main circuit does not have to be broken to fit it. Everything else in the circuit stays where it was.
A common mistake is to put the voltmeter in series, in the main loop. A voltmeter in series is wrong, so always check that it is across the component. Remember that an ammeter, which measures current, is the one that goes in series.
