* Voltage is a potential difference: Voltage is measured between two points in a circuit. It represents the potential energy difference per unit charge between those points.

* Measuring without affecting the circuit: An ideal voltage measuring device should only measure the potential difference without altering the existing current flow in the circuit.

* High impedance prevents current flow: A high input impedance ensures that the device draws minimal current from the circuit being measured. This keeps the voltage drop across the device negligible, allowing an accurate measurement of the potential difference between the points.

In simpler terms:

Imagine a voltage measuring device as a very narrow pipe connecting two water tanks at different levels. If the pipe is very wide, it will draw water from the tanks, changing the water levels and making the measurement inaccurate. A narrow pipe (high impedance) allows only a tiny amount of water to flow, minimally affecting the levels and providing a true reading.

Therefore, a high input impedance is essential for accurate voltage measurement because it minimizes the device's influence on the circuit being measured.