Kirchoff's Voltage Law states that the sum of the differences in Voltage (sic) between different points in a closed electrical circuit is zero. Example:
|-------A------/\/\/\/\-----B---|
|                               |
|                               |
P                               |
|-----C------/\/\/\/\/\/\/------|
With power source, P, and two resistors. The voltage of P + the change in voltage from A to B + the change in voltage from B to C = 0. You can set up an equation with the resistances ( R1 and R2 ), the current ( I ), and the voltage of the power supply ( V ):
V - I * R1 - I * R2 = 0