can also be defined as locus of points in a plane which are equidistant from a given point (the focus) and a given line (the directrix)."
The way it applies to us, is that the light that hits the parabola, will reflect back to one intersection point. That intersection point is called the point of focus. By placing the hot dog in the point of focus, all of the sun's rays that hit anywhere in the cooker, will reflect onto the hot dog...thus cooking it.Click here to see how the light reflects
By the way...the shape of the entire cooker is a parabolic trough.