Duplicating movieclips and hitTests

I was wondering if anyone could explain to me what the actionscript would be if i wanted to duplicate movieClips randomly from the edge of the flash? And how i would make the hit tests for them.

Looking at the tutorial
http://www.kirupa.com/developer/actionscript/duplicatemovie_mx2004.htm
I don’t get what
for (i=0; i<100; i++) {
means

and what the
_alpha
is in
{_x:Math.random()*300, _y:Math.random()*200, _alpha:Math.random()*50}

and does it automatically play the movieClip that you duplicated?