Why a voltmeter is said to be ideal when it has infinite resistance?
As resistance increases, current flow decreases.
If a voltmeter has low resistance then current will flow through the voltmeter and the voltage indicated by the meter will be wrong.
Having a very high or ideally an infinite resistance doesn't allow much or no current flow at all through the voltmeter. This helps to indicate the correct voltage between the points where the voltmeter is connected.
thats Why a voltmeter is said to be ideal when it has infinite resistance.