The resistor and the photo cell make a voltage divider. One side of the resistor is connected to +5v, one side of the photo cell is connected to Gnd. The opposite sides of both the photo cell and the resistor are tied together - and connected to the yellow wire. As the photo cell resistance changes, the voltage across it changes because of the resistor connected in series. If you changed where the yellow wire is, you'll be connected to +5 or Gnd. If you eliminated the resistor and connected the photo cell from +5 to Gnd, only the current through the photo cell would change. Adding the resistor to the circuit results is a change in voltage across the resistor, and across the photo cell. As the voltage across the resistor increases, the voltage across the photo cell decreases, and the inverse is true as well. The voltage across resistor and the photo cell will always equal the supply voltage: +5 volts. Hope this helps.