Kepler's Third Law of Planetary Motion states that the square of a planet's orbital period is proportional to the cube of its average distance from the sun. This means that planets farther from the sun take longer to complete their orbits compared to those closer to it. The law illustrates a consistent relationship between a planet's distance from the sun and its orbital speed. Understanding this law helps explain the dynamics of planetary motion in our solar system. Overall, it highlights the predictable nature of orbits based on distance from the sun.