The rotor is, electrically, a coil of wire wrapped around a slug of metal to form an electromagnet.
The electromagnet forms a magnetic field, the strength of which determines how much output the alternator stator coils can produce. (it also needs to be spinning).
The strength of the magnetic field is determined by how much voltage is applied across the two terminals. Usually, one terminal is applied to the battery NEG terminal and the other terminal gets various voltages depending on what the regulator determines is best for the battery.
That is why the rotor is usually tested for resistance as Pinhead pointed out. If the rotor coil is broken you will see voltage on one terminal and not the other. If the rotor coil is intact, you will see voltage on one terminal and not the other. So, the voltage test does not give useful information without also measuring the current going through the rotor, which means adding another meter for the test.
It is a very good test to know what resistance your rotor has. They usually fail by shorting the windings together. This lowers the resistance and reduces the alternator output, and further draws far more power from the charging systems output, robbing from the battery.