The planet speeds up when it's closer to the sun, and sweeps out a short fat triangle every hour. It moves slower when it's farther from the sun, and sweeps out a long skinny triangle every hour. The triangles have equal areas.
The only way I know to explain WHY this happens is:. That's how gravity works.
By the time you finish a couple of semesters of college Physics, you'll be able to take Newton's laws of gravity and motion, add some geometry and some calculus, stir well, and show why the planets must follow Kepler's laws.
A planet orbiting a star in a eclipse and sometimes it is closer to the star but sometimes it is farther. When it is closer the gravity on the planet from the star is stronger and it speeds up. The area the planet sweeps over is equal because when it speeds up the length covered along the orbital path is greater, but it is also closer to the star, and that dimension is decreased.
