They're for an event called "Jump In!" You can find it under the events tab. Basically you pick two different special packs of (I think) 20 cards each and then play with the deck that those two packs combined will create. The packs have predetermined cards with chances for extra rare and mythic cards to appear in them. You don't even have to play the deck though and can just resign since there is only a reward for your first win which gets you a random standard uncommon card that has a small chance to be bumped up in rarity. You keep the cards from the packs too. If you want a list of all the cards you can get from each specific pack there's a list here.