Don't quote me on that, but it appears to be some kind of a cheat device along the lines of the Exploder
"pipe" cards of the yesteryears - you put the original cartridge in the top slot, shove the device into your console and choose the cheats you want to use. Additional cheats could be uploaded via USB.
EDIT: Yep, it's an Action Replay MAX DUO, you can see it
here.