Kepler's First Law: The orbit of a planet will follow an ellipse around the Sun, which is at one of the two foci.
Consider a satellite in orbit around the Earth. A circle (or circular orbit) is a set of points that are a fixed, constant distance from a center point or focus (r = constant).
Now consider an ellipse, instead of one focus an ellipse has two foci. The sum of the distances from each foci to a point on the ellipse is constant (r1 + r2 = constant).