a question about codes of dynamic menus in JavaScript

I upload the snaps of the codes to Flickr.com.

Here is the html part: http://www.flickr.com/photos/59919989@N08/5470586866/

Here is the JavaScript part: http://开发者_如何学运维www.flickr.com/photos/59919989@N08/5470587034/

Could someone tell me what is the meaning of the circled part? What does it do? Thank you very much.

---

it gets the element with the id named "months" and in your html it shows that it is the select tag. it then sets the selected item by default using selectedIndex. 0 means the first item in your options list which is "Month"

also next time, please put your code in http://jsfiddle.net

---

Given that this is straight out of JavaScript & Ajax for the Web: Visual QuickStart Guide, 7th edition¹, what in particular are you having trouble with in the (line-by-line) explanation on pages 164-166?

_{¹ Co-written by me, btw.}