Optimization is a wonderful term that means making-things-go. Seriously. The whole process has to do with making the file sizes of your SWF files small and fast to download, splitting your SWFs into smaller content-specific SWF files so that users of your application aren't forced to wait until retirement for your application to download before it can be used, and (the most-often forgotten step) making your SWF files gentle on the end user's computer. That is to say, making sure it doesn't eat up all the available CPU and memory that it can find on the end user's system. Nobody likes a glutton (except for maybe the ancient Romans), so you want to make sure that your SWF file isn't gobbling up the technological buffet.
A well-organized, well-planned Flash application rarely needs much in the way of post-development optimization, although you will always have some minor tweaking to do. In some ways, the concepts of this lesson really ought to come first, but the idea of optimization without a context to display it in doesn't carry over well, so it is next-to-last in the book. That being said, take the lessons you learn here to heart, love them, nourish them, and they will serve you very well. When you embark upon a Flash project of your own, you must consider the lessons of optimization up front, in your planning, storyboarding, development process, and all through your testing. It's as important to your Flash work as skin is to your body.
So far, you have been following many of the best practices for building Flash applications without perhaps fully understanding why. For instance, you produced much of your content (news, home, and so on) in separate and smaller SWF files. You avoided using alpha tweens and overusing animation. You loaded in external image files. Now, let's start talking about why you did all that.