This is a very useful test to learn and is essential to fault finding on any electrical circuit.
When measuring resistance with a multimeter they only send out 1.5v and as such do not test a cable fully , doing volt drop testing uses the actual voltage of the supply so tests the cable fully and will easily detect internal damage or resistance within the cable.
In voltage drop testing you test one side of the cabling either positive or negative, and measure on a loaded circuit for example touch multimeter lead from positive of battery to starter motor positive post and measure the difference in voltage across the two pints this is the amount of volt drop on the cable.
this need to be as low as possible it he millivolt range if its 1or 2v or more its a problem there is damage or a break in the cable creating a higher than normal resistance and using up some of the voltage.
This can be done for any circuit on both positive side and ground side of any load and will tell you if you have damage or high resistance to the cabling.
The ground side is the same test touch from negative terminal of battery to negative terminal of load device in the case of a starter it would be the metal body of starter as this is earth to ground via the engine, the reading on the ground side should be 0v as the load should use up all voltage on the circuit but it reality there will always be a little voltage due to cable and real world results.