They described them briefly in the GenCon podcast (
Dungeons & Dragons Roleplaying Game Official Home Page - Article (D&D Podcast: D&D Preview Show)) at 37:55. The general gists that I took from them were:
What they are:They are elements of fortune or luck that affect how your character is acting and add randomness to the game.
They emulate in movies when a random twist or sudden change in how the combat is flowing that results in an interesting affect.
They are not pure power-ups. They wanted them to be rare random events that could affect a fight and make the combat take a turn that was not expected.
They are cards in booster packs, like Gamma World booster packs or Magic booster packs.
They can be used in organized play events (like Encounters) or in home games.
How they work:You draw a card and if you meet the conditions on the card you can use it for some benefit on your turn.
They can define what sort of thing your character is going to want to do on their turn. (I assume so you can meet the conditions so you can use the card.)
How they buying them:They are "totally optional".
Players are not meant to buy an "optimized 60 card deck that destroys everything" or "buy a case and collect them all".
Collecting them all defeats the random surprise element of them if you know what cards there are. In playtesting they were the most fun when they are unexpected and are a surprise to everyone at the table.