Can you explain what's these code mean?

1

Can someone explain the differences between these code for me please?

thanks

_root.image_mc._x = _root.image_mc._x + (Stage.width / 2 - _root.image_mc._x) * speed;
_root.image_mc._y = _root.image_mc._y + 100 * speed;

and

_root.image_mc._x = _root.image_mc._x + (Stage.width / 2 - _root.image_mc._x) * speed;
_root.image_mc._y = _root.image_mc._y + (Stage.height / 2 - _root.image_mc._y) * speed;