Basically they're just Miis that were sent out by Nintendo (as opposed to StreetPass, which is when you pass someone with a 3DS on the street). Nintendo sends some Miis out during E3 each year (and occasionally at other times too).
It's basically Nintendo's way of sending stuff out to 3DS users, whether it be a Mii or a notification. I believe it can also be used to download from the eShop.