What does voltage drop mean in a circuit?

I answered this SE EE question an hour ago.

My answer

You can get a good intuitive idea of ​​what a "voltage drop" is from Ohm's famous experiment in 1826 where he studied the voltage distribution along a resistive wire. Also, fluid and other analogies that allow us to transfer our ideas from life to abstract electrical phenomena, are very useful. See, for example, the Wikibooks story about the voltage drop phenomenon that my students and I created in 2008 where we investigated voltage drops across various resistive materials.

In 2013, I dedicated a special question in ResearchGate to "voltage drop" compared with "voltage"; I hope it will be interesting for you.

You also asked: "How can I use a single resistor to drop the voltage to 3 V?"

You can... but a more sophisticated trick is to use a "non-linear voltage-stabilizing resistor", e.g. a diode with 2 V forward voltage. And since there is no such a diode, you can make one by connecting three 0.7 V silicon diodes in series. I hope you understand the benefits of this solution.