Ion-selective electrode: high input impedance to measure the voltage
In your setup, the assumption is that both E and Ri are unknown.  You are trying to measure E with a voltmeter having an input resistance of RL. The voltage seen by the voltmeter, call it Ev, will be the result of the voltage divider formed by Ri and RL, namely Ev = E*RL/(RL+Ri). Since Ri is not known, E cannot be accurately determined unless RL is much greater than Ri. In that case, the equation reduces to Ev = E. That is why your reference calls for a high input impedance voltmeter.