Dumb q - What's a []

Hi
My first post here so let me get started ^^.
I’m sortta familiar with a.s., developing an load-from-xml image galery with thumbnails and now i’ve come to build the preloader.

Well i found some example of what i must do (in this forums actually). But i got pretty clueless when checking the code i saw “[]”:

var imageData:Array = [];

I know so far that sets imageData to be an array of [], but what’s a []? Oo