I am not from an engineering background, so this may be very simple and stupid but:
Voltage is the difference in electric potential between 2 points
Voltage drop describes the reduction in energy of the supply through a passive element, given by the amount of current through the resistance of the element
So for the 9V seen on the diagram, there is a difference in electric potential between the 2 terminals of the battery. the difference is 9V which means 9Joules per 6.242×10^18 electrons (unit charge) can be given to move from one terminal to the other terminal?