Nintendo UK is in the middle of a national tour around shopping malls, promoting current and upcoming releases on Wii U and 3DS. One promotion it's been running at events has been to give out free codes for Photos with Animal Crossing, an AR app for the 3DS that allows you to snap pics with some of your favourite animals.
It's the same basic concept as the AR cards you receive free with your system, most directly familiar with the previous limited-distribution Photos With Mario app.
Nintendo UK has decided to send copies out to plenty of 3DS owners that aren't attending any of its tour events. If you're on the Nintendo Newsletter mailing list you may have the following email waiting for you with a download code.
All of this is partly in promotion for Animal Crossing: Happy Home Designer, of course, which has been a sales success so far in Japan and arrives in the West soon.