It's a "passme" device/ROM.
Say, for example, you want to run Nintendo DS Software off a GBA cart storage device... you need a "passme" device.
What it does, is tells the NDS to boot up an ".nds" file from the GBA cart storage device, as opposed to the main ROM slot. So, it's like a boot disc relly.
The "max media launcher" is actually a "passme" device too.
There are many "passme" devices out there, along with many GBA cart storage solutions (GBAMP, Supercard etc... etc...)
Not all "passme" cards work with all GBA cart sstorage devices...
...furthermore, not all software is compatible with all GBA cart storage devices.
For that reason, I have created this thread, which tells you what to buy, and where from. It all works together nicely:
http://www.cliosport.net/forum/showthread.php?t=169945
As an alternative to a "passme" device, you can flash the firmware on your DS so that it boots .nds files.
In order to load the firmware update, you'll need a "passme" device, so you might as well not bother (particularly when there are dodgy firmware releases out there that will render your DS useless).
I personally did sucessfully flash my firmware, so that I no longer have that annoying health & safety warning on startup. A bonus is that I no longer need my "passme" device to run my software.