As you know, when a resistor is connected across the positive and negative rails of a power supply, some voltage will be dropped through the resistor. This is due to the resistance offered by the resistor. Let us see one example.
Voltage drop means the less voltage from the resistor than the supply voltage. It is the supply voltage minus the voltage across the Resistor.
For example, two resistors R1 and R2 are connected in series across a 12 volt power supply. R1 is 10K ( 10,000 Ohms) and R2 is 1K (1000 Ohms).
Now the Current through the series resistors
I = V / R1+R2 = 12 / 11,000 = 0.00109091 Amps
Now VR1 = IR1
VR2 = IR2
Voltage drop across R1
VR1 = IR1 = 0.00109091 Amps x 10,000 Ohms =10.9091 volts
Input voltage – 12 V
Voltage drop across R1 – 10.9091V
Voltage output from R1– 1 V
Note that slight difference of 0.6 volts. It is because the input supply is 11.7 volts
Note – Here R1 and R2 are reversed for measurement
So total voltage drop across R1 and R2 = 10.90909 +1.09091 = 12 Volts
That means the 12 volt supply voltage splits in to two through R1 and R2 and finally the total values reach same as the supply voltage. That is, the voltage drop in one resistor is compensated by the voltage drop in the other in series connection.