What is this about? Is this like the iron swords thing before with crystals, but with keys and chests?

Sort of. Basically you will collect keys to open the chest. The chest has a set amount of cards in it. Everytime you open the chest with your keys you draw 1 card. The chest now has 1 less card in it. The cards in the chest are preset, so the more you open it the more likely you will get the different cards.

EX: 60 Cards in the Chest
You open it 59 Cards left
Etc Etc.

You can reset the chest whenever you'd like back to 60 cards to try and repull cards you may have already gotten.

So basically we can only get this Chest with GC?

No, you will be able to obtain Keys without GC. But you can obtain more a lot faster with GC.
Check in game for more details.

No no I mean how do we use the keys? A lot of people are on message board are asking...

Do we have to buy something to use the keys?

Random Boss chests can sometimes contain keys as I've just discovered.

Thank you for the responses.

Click on the banner for immortal chest. If you have three keys you can unlock the chest and pull our a gift (immortal). Weird thing is, I have at least one key and it still shows zero. I'll wait until I have red and see.