You’re familiar with the flash carts that Nintendo is waging a questionably successful war on, yes? Basically they just read from a ROM file and tell the console or handheld what it would hear if that game were actually in the slot. They work quite well and pirating games is unbelievably easy, so you can see why Nintendo would be worried. For the SNES, though, I doubt there will be quite as big of a response.
This cartridge does the same job but fits into your SNES, allowing you to load it up with as many ROMs as you can fit on there. You add ’em on there via USB (the cart has 256MB of space, which will probably fit about 100 games) and then use the on-screen menu to navigate. It doesn’t seem to offer any of the advanced features of some flash carts — freeze states, rewind and fast-forward, that sort of thing. It also won’t support some games that had added hardware, like the SuperFX chip in StarFox and Stunt Race FX. It costs $169, which sounds expensive until you think that there are probably only a thousand people in the world who want one of these.
I personally have no use for this thing; I have all the cartridges I need, which is to say Super Mario Kart, Zelda, and Street Fighter II Turbo.