from the site itself:
"SuperKey for the Nintendo DS and DS Lite is the perfect solution for accessing DS media from any GBA flash storage device. Simply snap in the SuperKey cartridge, and that's it! Media, freeware, executables and other files stored on your flash card can be launched. The Flash Cart revolutionised the GBA home brew scene, and with SuperKey, it will do for the DS too.
SuperKey is compatible with most types of flash cards including SuperCard, M3 and NEO.
Feature:
# . Access media from any GBA flash storage device
# . Use home brew GBA software on your DS
# . Plug-and-play ease of use
# . Compatible with most types of flash card
# . Original Size
# . NDS, NDSL and IDS support
# . Sleep mode support
# . Whitout any flashing of firmware. Without any setting. Just plug any play with SuperCard.
# . The best and best partner of SuperCard, not like SuperPass. SuperKey do not need to turn off and turn on NDS any more."
so i guess basicly it makes the Super Card compatible with the DS
as for what the Super Card actually does, it's hard to say as the site is ambiguous. the translation is rough, but here's what i inferred. i cannot guarantee that any of this is true or accurate, but i'm fairly confident in most of it:
-the supercard goes into the GBA cartridge slot.
-it acts as an adapter to allow you to insert SD Flash Memory cards into the NDS/NDSlite/GBA to display pics, play vids, and play games (that can seemingly be downloaded from the internet for free instead of buying them???) or even home made games.
-it can function as a game cheat code device
-it can save your game at any point that you choose and you can resume there later instead of having to wait for a save point
-with regular use, it can help prevent future outbreaks of herpes, though you still may be able to spread it to your partner. minor side effects include diarrhea, nausea, and headache. in rare cases, some people have experienced a severe heart condition. only you and your doctor can decide if Super Card is right for you.