WK 4 Practical Experience: FLASH (post #2.75/more notes)

I really do not feel like I fully understood the Adding Interactivity segment.  I followed everything that the instructor was saying - but I did not understand why I was doing it.  My exercise file worked as it was supposed to upon my completion and I get the basic idea of what I did.  I have a VERY basic understanding of writing code so some of that part makes sense, its just the putting it all together.  There was a promise that the code part would be explained in more detail later but this is what I am up against now:

adding interactivity