This was an interesting little device, released only in Japan. The unit came with a controller/interface (I believed), that plugged into controller port 2, which was attached to the unit that scanned the barcodes. The controller/interface had an LCD screen, and some buttons I do not know what they do.
The idea was, you took barcodes off various products, anything with a UPC barcode, placed them on cards that come with the unit, and run them through the barcode reader. Based on the information in the barcodes scanned, your players would be given various attributes. Using these attributes, a battle would take place between the two players. From what I have read, this accessory, originally released for the GameBoy, was very popular in Japan. There was even a run on a soup (in Japan) who's barcode was rumored to give a player amazing power.