It's a doodad that attaches to the back of the Jag and offers an assortment of AV options and such, similar to something like a TurboBooster but with networking options.
Honestly, I'd be as happy with the Catbox as the Scatbox, issues of rarity and price aside. That thread on AtariAge is fairly absurd, what with its hypothetical discussions of hardware durability and aesthetics - neither the Catbox nor Scatbox are beautifully designed.