To help quads, helicopters, etc. fly well, we want to eliminate vibration and to also have the motors run evenly and smoothly. One thing that helps with that goal is balancing the props (propellors). Props are mass-manufactured, and they’re not perfectly even on both sides. That means one side will be slightly heavier, so when they spin, they won’t spin perfectly evenly, which can cause vibration. Here’s info about how to balance the DJI Phantom 2 props.