Kepler's second law implies that the closer a planet is to its star, the faster the planet travels. This law is related to conservation of angular momentum. The law is important because the planets' orbits around stars are not circular, so planets are closer to their stars at certain points.
Kepler's second law assumes that if a line is drawn between a star and its planet, for any set time period, that line sweeps out the same area each period, no matter where in orbit the planet is at the time.
For example, if the time period is 10 days, the line between the planet and the star sweeps out equal areas every 10 days. This is possible because when the planet is closer to its star, it moves faster, and when it is further away from its star, it moves slower. At aphelion, the point in a planet's orbit furthest away from the star, the planet moves the slowest. At perihelion, the planet is closest to its star and moves the fastest. This is because of the planet's conservation of angular momentum.
Angular momentum is directly proportional to an object's moment of inertia and angular velocity. When a planet's distance from its star increases, its moment of inertia increases. For its angular momentum to remain the same, the planet's angular velocity has to decrease.