What's the meaning of "氪金"? How to translate it? I know the meaning of 氪 is krypton. But I find there are many people say: 这个游戏真好玩，我要氪金. 
Now they have got the game and played it, so they needn't buy the game now, even sometime they say so when play a free game. So how to understand the meaning of it?
氪金 means pay, charge.


  氪金，原为“课金”，指支付费用，特指在网络游戏中的充值行为。


Even the players are playing a free game, they still could pay money for items in games (to become more powerful, etc).

Also see here.