Drag the point F and scroll the point C along the red line (press Ctrl + Shift + F to clear the screen).
If you locate a point on a sheet of paper and choose an edge of it, then the loci you get by folding the paper in order to make the poin meet the edge you chose, is known as "Parabola". In thsi context, every fold on your paper is equivalent to the perpendicular segment through the midpoint between F and C.