That's your Super Smash Bros. Brawl Friendcode, then.
Go to the icon on the bottom-right of the Wii menu, then select the "Compose new message (?)" button; from there, choose "Address Book," and it will list your Wii friendcode; flipping through the pages of the address book lets you add others' Wii friendcodes as well as confirm that they have added you back (those who have added you back have a black name in the book, those who have not have a grey name).
It's lame, I know, but if you don't add someone's Wii Friendcode then you can't send messages to them, you can't send Miis to them, and in games like Brawl you can't send custom stages to them.