A voltemeter is a device used to measure the amount of potential difference across a resistor or a combination of resitors.
The resistance of an ideal voltmeter should be infinite.
This is because a voltmeter is connected in parallel in a circuit across resistor/resistors.
If the resistance of the voltmeter wont be high, it would draw some current and the other resistors connected in parallel with it will not get enough current through them.
Hence, in order to avoid the change of current flowing through the resistor/resistors.
The resistance of a voltmeter should be infinitely high, so that it does not draw any current.