With a voltmeter, you measure a voltage between two points in a circuit...
For measuring voltage you add the voltmeter using two nodes in the circuit. The to be measured voltage applies now to the voltmeter. This is a direct consequence of the Kirchhoff's Voltage Law (KVL). Now, you read the value of the voltage in Volt [V] at the voltmeter.
But pay attention to two points...
An ideal voltmeter has an infinite high internal resistance (Ri = ∞ Ω).
No additional current flows through the voltmeter and therefore has no influence on the measurement result.
An ideal voltmeter is unrealistic. But what is a real voltmeter and what influence does it have...?