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Think of an orbit's two focus points, or foci, as its centers. In a perfectly circular orbit, they are at the center of the circle. Most orbits follow the shape of an ellipse, and have two foci.

Kepler's first law states that a planet will orbit the sun in an eliptical shape, and that this orbit will have two focus points: internal points that help determine the shape of an ellipse. The distance from one focus to any point on the ellipse and then back to the second focus is always the same. The sun will sit in one of these focus points.

An orbit that is more eccentric or stretched out (like a comet's) will have foci that are farther away from each other a more circular orbit, like the Earth's orbit.