Kepler's laws are about the planets' orbits. The first law says that the orbit of any planet around the Sun is an ellipse with the Sun at one of the focii. The second law states that the line joining the Sun to a planet covers equal area in equal intervals of time. The third law states the square of the time period of the planet is proportional to the cube of the semi-major axis of its elliptical orbit. Kepler's law explained planetary motion, but nothing other than motion, and really no one could properly explain why Kepler's law worked. So therefore we move on to Newton's first law.