That means someone probably had to circumvent the system's anti-piracy measures.
Someone decided that the current selection of titles for the DS wasn't enough, so they had to go and make their own game for the system. The first homebrewed DS game is a version of Tetris. It really doesn't mention anything other than that, but one of the screenshots does show it running on both screens.
For this to be possible, the encryption scheme or other anti-piracy measures Nintendo has in place to protect its DS games has probably been compromised. Nintendo probably didn't envision a computer cable wired into the Game Card slot when it came to playing games on the thing, after all.
It's still neat to see other people tinkering with the system and see what they can do with it. Who knows, in the future, maybe some crack homebrewer can come up with something that uses the DS's features in a way not even Nintendo could dream up.
Link: http://www.themodgods.com/2005/02/fi...ame-for-ds.htm