Kepler's Second Law, also known as the Law of Areas, states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. This means that the area swept out in a specific time period is constant, regardless of where the planet is in its orbit.
The factor that remains constant in this motion is the areal velocity of the planet, which is the rate at which area is swept out by the line connecting the planet to the Sun. This concept reflects the conservation of angular momentum in the gravitational interaction between the planet and the Sun, meaning that as the planet moves closer to the Sun (periapsis), it moves faster, and as it moves farther away (apoapsis), it moves slower, ensuring that the area swept out remains constant over time.