What this nonsense is about is, every edge segment borders at most two fills. When rasterizing the object, it's pretty handy to know what those fills are ahead of time, so the swf format requires these to be specified.
swfshape->setleftfill() sets the fill on the left side of the edge- that is, on the interior if you're defining the outline of the shape in a counter-clockwise fashion. The fill object is an SWFFill object returned from one of the addFill functions above.
This seems to be reversed when you're defining a shape in a morph, though. If your browser crashes, just try setting the fill on the other side.
Shortcut for swfshape->setleftfill($s->addfill($r, $g, $b [, $a]));.
See also swfshape->setrightfill().