Question about Filter Effects tutorial?

I am a noob to Actionscript. I was doing the Flash 8 Filter Effects tutorial located here It is a very good tutorial and it works great. I am determined to know why it works though so I read through each line of code. There is one line I do not understand the purpose of

it is
<code> this.filters = [gf]; </code>

it comes right after the rollover event, I am confused because it seems like the filter was called by the kText movie clip. In the beginning of the movie.
Like I said I am a noob its probably a obvious reason, could someone point it out to me please.