A Julia seed is a pair of numbers which correspond to a point on the Julia's corresponding non-Julia fractal. This pair of numbers uniquely defines a particular Julia set fractal. The Julia seed is used in the Julia calculations as the value for C in the equation Z = Z + C, while the pixel being calculated is used as the initial value for Z. The Julia seed can be adjusted and viewed by using the Set Julia Seed window.
As the Julia seed approaches and crosses the edges of the non-Julia fractal associated with the current Julia set, the Julia set changes dramatically. When the seed goes inside the interior of the non-Julia fractal, the Julia set is ripped apart by an explosion of the interior colour at its center. When it goes outside of the non-Julia fractal, the Julia set instantly collapses in on itself, producing dramatically different images.
The mathematical details are not important for an appreciation of Julia sets.