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. 

[FONT=Courier New]1 var btnNum:int = 5;[/FONT]
[FONT=Courier New]2 var spacing:Number = 10;[/FONT]
[FONT=Courier New]3[/FONT]
[FONT=Courier New]4 var navBar:Sprite = new Sprite();[/FONT]
[FONT=Courier New]5 addChild(navBar);[/FONT]

[FONT=Courier New]6 var btn:SimpleButton;[/FONT]
[FONT=Courier New]7 for (var i:uint = 0; i < btnNum; i++) {[/FONT]
[FONT=Courier New]8     btn = new Btn();[/FONT]
[FONT=Courier New]9 = "button" + i;[/FONT]
[FONT=Courier New]10    btn.x = spacing + i * (btn.width + spacing); //do not understand                                                     // this section[/FONT]
[FONT=Courier New]11    btn.y += 5;[/FONT]
[FONT=Courier New]12    btn.addEventListener(MouseEvent.CLICK, onTraceName, false, 0,[/FONT]
[FONT=Courier New]      true);[/FONT]
[FONT=Courier New]13    navBar.addChild(btn);[/FONT]
[FONT=Courier New]14 }[/FONT]
[FONT=Times New Roman] [/FONT]

The section which explains line 10 is as follows:

[FONT=Times New Roman]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 ([/FONT][FONT=Courier New]spacing[/FONT][FONT=Times New Roman], plus zero times the sum of the width of the button and [/FONT][FONT=Courier New]spacing[/FONT][FONT=Times New Roman]). 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.[/FONT]

From the above quote, i understand that the button the first button is placed horizontally, but i don’t understand what is after it. Can somebody explain what the paragraph means and how it does it’s position please. Because if i want to do something similar i have a good understand of how to start it.

I’ve given the fla so you can see what it is referrring to. If any of you guys like me have the Learning Actionscript 3.0, A Begginners Guide, book it is on page 69, Chapter 4, about halfway down the page

Sorry if i am asking that is low level, i’m still learning it and would like an expert opinion on it.