Oil droplets were sprayed and allowed to fall through a small hole in the apparatus, where they could be viewed with a light source. As the droplets fell, they aquired electrons that formed from the interaction of high energy radiation with the air. The charged droplets then interact with the electrically charged plates of the apparatus, and since negative charges repel each other, by having a negatively charged plate at the bottom of the apparatus allowed the fall of the drops to be slowed down, stopped, and even reversed. Using the mass of the drops, and the voltage required to stop the drops, Millikan was able to calculate the charge of each drop, and then find charge of each electron. (-1.6*10^-19 C)
The magnitude of the charge is important because it determines how stronglly an atom holds it electrons, and therefore this number plays a huge role in the interaction and sharing of electrons between compounds as well.