? as?

1

i’m working through a screen handler tutorial( VERY HELPFUL, nicely written, and I’m learning so there u go)http://active.tutsplus.com/tutorials/workflow/handling-screen-architecture-the-painless-way/

in this tutorial lives this chunk of code:

private function removeOldScreen():void{
    var oldScreen:MovieClip;
    oldScreen = screenLayer.getChildAt(0) as MovieClip;
    screenLayer.removeChild(oldScreen);
}

my question is “When I use the “as MovieClip;” part what am I saying?”.