Final answer:
When a voltmeter reads 120 volts with its leads on either side of a switch, it indicates that the switch is closed.
Step-by-step explanation:
A voltmeter measures the potential difference between two points in an electric circuit. In this case, when the voltmeter reads 120 volts with its leads on either side of a switch, it indicates that the switch is closed. When the switch is closed, the circuit is complete and an electric current can flow, resulting in the potential difference measured by the voltmeter.