What you need is a DS with the game, and a Wireless Access Point with a route to the Internet. Read more about it here (Wikipedia), or here (Nintendo). A friend code is basically a registered 'key' that is a mashup of the serial number of your DS and the serial number of the game. This way, you can register your mates friend codes on your DS and it will know if they are online at the same time so you can play each other. You can also play with random people from all over the world.