This simulator demonstrates Johannes Kepler's laws of planetary motion, particularly the first two laws:
Observe how the planet moves faster when it's closer to the sun (at perihelion) and slower when it's farther away (at aphelion). This demonstrates Kepler's second law.
The elliptical shape of the orbit showcases Kepler's first law. In reality, most planetary orbits are much less eccentric (more circular) than shown here, but the principle remains the same.