The latest freebie that's been released for the game, and is a universal code that will work on all platforms. Expires next Monday, 27 July. It will require a SHiFT account to redeem, but you don't need to own the game to collect them.
Golden Keys allow you to unlock the special weapons chest on the spaceship Sanctuary, and will be an item that's suitable for your character at the level they are when you open the chest. It has increased chances to get a Unique or Legendary item. Thus, a lot of players will hoard their keys until they hit the level cap in the hopes they'll get a sooper-dooper-ultra-rare weapon; in practice, it really only means your chances improve from 'vanishingly slim' to 'highly improbable', and most of what you get is best to sell back to Marcus to generate income.