A voltage drop test measures the amount of voltage lost in a working circuit – important because a circuit may have the ability to carry voltage, but not amperage, and subsequently may not operate correctly (Google "Ohm’s law").
Voltage drop measures from the most positive point in the circuit to a negative point in the circuit (not just simply from positive to negative). Placing the meter’s positive lead on the positive post of the battery, and the negative lead on the starter motor’s positive connection is an example of a voltage drop test (if the circuit is operating).
It’s an easy test.
Find the most positive point in the circuit and place the meter’s positive lead there.
Then place the negative lead further down the circuit. Turn the circuit on, and check the meter for the reading.
Generally, there shouldn’t be more than 0.5 volt drop in any section of a circuit.
Faulty circuits usually show more than a full volt lost in the circuit, not just 0.3 or 0.4 volts (quite often it’s four volts or even higher).
Tip: use the "record" feature of the multimeter to capture voltage readings as the circuit operates.
Once you’ve isolated the section with the greatest drop in voltage, you’ve found the problem.