WDTM: event.stageX - circle.x

programming flash
1

WDTM: What Does This Mean, i do not want to write long title :look:

i tried to understand the following code. it is drag and drop target.
below, i did not understand these lines:

offsetX = event.stageX - circle.x;
offsetY = event.stageY - circle.y;

what i figured out is the var offsetX store the stageX coordinate. but why there is " - circle.x". i deleted the " - circle.x", i noticed that the circle jump when i drag to position itself in center of my mouse.

//complete code

var offsetX:Number;
var offsetY:Number;

circle.addEventListener(MouseEvent.MOUSE_DOWN, startDragging);
circle.addEventListener(MouseEvent.MOUSE_UP, stopDragging);

function startDragging(event:MouseEvent):void {
    offsetX = event.stageX - circle.x;
    offsetY = event.stageY - circle.y;
    stage.addEventListener(MouseEvent.MOUSE_MOVE, dragCircle);
}

function stopDragging(event:MouseEvent):void {
    stage.removeEventListener(MouseEvent.MOUSE_MOVE, dragCircle);
}
function dragCircle(event:MouseEvent):void {
    circle.x = event.stageX - offsetX;
    circle.y = event.stageY - offsetY;
    event.updateAfterEvent();
}