You can easily try it out for yourself. Place a loop of string around two pins stuck through a piece of paper into a drawing board. Use a pencil to keep the loop taut. If you move the pencil carefully, it should trace out an ellipse. The pins correspond to the blue points and the pencil corresponds to the black point in the animation.
Every ellipse has such a pair of points.
The foci are unique. There are always precisely two of them. A taut loop of string of appropriate length stretched about the foci as shown in the animation will draw the ellipse. The Game Your mission , should you choose to accept it, is to find a focus. You will be given an ellipse of random dimensions and you will have to try to click on one of the foci.