What does it mean for a planet to sweep out equal areas in equal times?

Kepler’s second law of planetary motion, derived by the German astronomer Johannes Kepler, states that a radius vector joining any planet to the Sun sweeps out equal areas in equal lengths of time.12

This principle describes how a planet's orbital velocity changes as it travels along its elliptical path. Because the planet must cover the same amount of area in the same amount of time, it must move faster when it is closer to the Sun—at perihelion—and slower when it is farther away—at aphelion.13

Essentially, this law demonstrates that the angular momentum of a planet remains constant as it orbits the Sun. It was a foundational discovery that later assisted Sir Isaac Newton in formulating his law of gravitation.12