defining a "document ready handler"

I am trying to use a simple slideshow in my page but got stuck when it tells me to ‘define a “document ready handler”‘.
I was going to use this slide so i don’t have to know too much about javascript but got stuck.
can someone tell me what he means by it and what i need to do?
here is the source page: [url]http://tobia.github.com/CrossSlide/[/url]
if you scroll down, on the bottom under #4 is where i got lost.
Thanks

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