by "cart", I believe they mean a cartridge, like a Game Boy Cartridge. It's the rectangular thing you put in a game boy, and each one contains a different game. They are saying that in order to play the game fully, you need to buy the Game Cartridge of Pokemon Emerald, and play it on a real gameboy (not VBA)
you can always check online sites, using google or amazon, to find places to buy emerald. The reason you would want a cartridge is so that you could play the whole game (like the battle frontier), and because the makers of pokemon deserve the 30 bucks you'd be paying, since they did spend a lot of time on the game, programming it and such.
Go to "https://www.amazon.com/exec/obidos/tg/detail/-/B0007D4MVI/sr=8-1/qid=1144218863/ref=pd_bbs_1/002-4110984-7916869?%5Fencoding=UTF8&v=glance" to purchase the real game. Aplologies if I'm not supposed to post a link to an outside source. I DID read the rules though lol :)
