Voltage drop happens when there is resistance to current flow.

A light bulb has resistance and drops all the voltage across its filament - heat is created and light is produced.

To imagine how resistance works, let's use a water hose.

If you pinch a water hose, not as much water will go through it, but pressure and turbulence are created at the point of the restriction.

Electricity flows through wires sort of like that, but all the electrons flow on the outside of the wire, not through the middle of it.

 The fuse, switch, and motor in the above schematic drawing are wired in series...

  An automotive circuit starts and ends at the battery.  Electronic theory states that electrical current flows from negative to positive because electrons are negatively charged.

If you connect a small piece of wire from one battery terminal to the other one, that wire gets very hot and melts, usually in two - don't hold a wire in your bare fingers and do this, and don't make sparks around a battery, because hydrogen gas is created in a battery and tends to explode.  Always wear safety glasses when working around an automotive battery!

 The point is that the wire becomes the load.

 When a resistor is wired in series with a load, the load receives less voltage - this principle is harnessed to give you your various blower motor speeds... when the motor is on low, the current is flowing through 3 resistors. On medium low, two resistors. On medium high, one resistor, and on high it's hard wired, bypassing the resistors.  Since resistors get hot, the blower resistor is mounted in the air channel to keep them cool, and they are outfitted a small built in thermal fuse that will blow if the air stops moving but current is still flowing.  

  When a starter doesn't work right even after it has been replaced, it's a good idea to check for voltage drop on both sides of the circuit.  The positive side of the circuit shouldn't read more than .5 volts of drop and the negative side shouldn't read more than .1 volt of drop.  The drawing below represents both tests - the ground side and the power side.  If either side is high, clean the connections. You can also do a voltage drop test on the charging system circuit by going from the battery to the large terminal on the alternator.