Short, and tied to the tests that use them. Read one on the way to the bay.
Voltage drop is the voltage lost across a section of the circuit while current is flowing through it. It is the fastest way to find a bad connection on a live vehicle.
Every volt lost in the wiring is a volt the load never gets. Measure across the suspect section, with the circuit running.
Put the probes on either side of the section you suspect, with the circuit loaded and running. What the meter shows is what that section is stealing. A good cable or ground shows almost nothing. A corroded one shows the fault straight away, without disconnecting anything.