ActionScript 3.0 :: Paragraph Explanation - Dynamic Nav Bar

I have been reading Learning Actionscript 3.0 for about 1-2 months now on and off and i have stumbled across a paragraph that I don't quite understand concerning the position of a navigation bar buttons.

Code:
var btnNum:int = 5;
var spacing:Number = 10;
var navBar:Sprite = new Sprite();
addChild(navBar);
var btn:SimpleButton;
[Code] .....

The current button is positioned horizontally (line 10), offset by the spacing set in line 2, plus the width of the button and another spacing gap for each button in the loop. Therefore, the first button is positioned only 10 pixels to the right of the container's registration point (spacing, plus zero times the sum of the width of the button and spacing). The last button is positioned 10 pixels to the right of the container's registration point plus 4 times the width of the button and spacing. The vertical position is also set, moving the button down 5 pixels. From the above quote, I understand that the button the first button is placed horizontally, but i don't understand what is after it. What the paragraph means and how it does it's position.