Plotting points a constant arbitrary distance from 2 focal points to generate an ellipse.
If you envision a string being stretched around two points, the shape outlined by the string would be an ellipse. Note as 'a' and 'b' change, the length and width of the ellipse also change. Enable "Trace" on the point to actually see the ellipse.
The above applications only demonstrate the creation of horizontal ellipses, the orientation of an ellipse can vary however.