I am watching a lecture on circuit analysis and the instructor is talking about voltage drop across the circuit. He hasn't really explained what it is.

My understanding based on these lectures was that voltage refers to the strength of the power source (e.g. a battery), and that the actual movement of electrons is the current. The current doesn't vary because it has to form a continuous chain - but if the voltage drops in the way he is saying, how can the current stay the same?

He gives an example of a 10 Volt power source hooked up to a 10 Ohm resistor. i = V / R means that i = 1 Ampere. I understand the mathematical relationship fine, I just don't get what he means by 'voltage drop' at all.

https://youtu.be/lf0lMDZVwTI?t=680