What did Newton add to our understanding of Kepler's laws?
Newton's work on gravity introduced the mechanic for the motion of the planets.
Kepler derived his laws of planetary motion from the vast amounts of data collected by Tycho Brahe. Brahe's observations were accurate enough that he was able to derive not only the shape of the planets' orbits, but their speeds as well. Kepler believed that some force from the sun pushed the planets around in their orbits, but he was unable to identify the force.
Nearly a century later, Newton's work on gravity revealed why the planets orbit the way they do. When applied to the planets and the Sun, Newton's law of universal gravitation accurately predicts the motion of the planets.