I wondered the same thing and I found this YouTube on how it applies to diodes...Alex is free to answer and remove my link. I just thought I would try to help..

https://youtu.be/M7tmFbIoVLM

Voltage drop the way Alex is showing it is the measurement across a FET's terminals in diode mode, with him showing you the correct readings and incorrect (shorted).

If you use a voltmeter to check voltage across a resistor that has current passing through it, that is another thing altogether.