Kepler's Second Law states that a planet revolves around the Sun in such a way that the areas swept by the line between planet and Sun in equal times are always the same. (Area A is the same as area B.) In other words, the planet has to move faster when it is close to the Sun, slower when it is further away.
In reality the excentricuty of the orbits of all planets is much smaller than show here, and their ellipses are much closer to circles, which made it possible for centuries to believe that planets move on circles.
Web site of the Tiverton Astronomical Society, http://www.tivas.org.uk/solsys/tas_solsys_phys.html (accessed 4 February 2004)