According to my textbook, it is said that, for an ammeter:
It is essential that the resistance $R_A$ of the ammeter be very much smaller than other resistances in the circuit. Otherwise, the very presence of the meter will change the current to be measured.
Which makes definite intuitive sense to me. One would hope its resistance is very low, because otherwise it would be like trying to measure the speed of a car by putting spikes under its tires just before.
However, what it says for voltmeters is not intuitive for me:
It is essential that the resistance $R_V$ of a voltmeter be very much larger than the resistance of any circuit element across which the voltmeter is connected. Otherwise, the meter alters the potential difference that is to be measured.
Why would it having a small resistance cause a hamper on anything? Why does restricting the current of charge carriers a necessity in measuring potential difference in the circuit? This is coming from someone who is very unfamiliar with circuits in general, but trying to learn incrementally.