It means "the base" and was originally a register of fighters from Saudi Arabia who went to fight in Afganistan against the russians. It was so they could keep track of the dead and wounded.

At least thats how I remember it being described by Arab commentators who couldn´t understand what all the fuss was about when "al whatever" was suddenly public enemy number one.

The "terrorist" organisation is a fabrication of the C!A or similar agency, to scare the american sheeple into giving up their rights. And it worked.......