They're what await players of Duels of the Planeswalkers 2013. When a player first launches the game (or goes into the Extras tab), they'll get their code. And when they enter that code on our website, they'll get a voucher to print out. Then they'll take that voucher to a participating store and get a booster.

http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/arcana/994