You're so damn good. 666 is a Chinese slang that is widely used to praise someone when they pull off something impressive, or as a sarcastic remark when someone makes an epic fail. The more 6 you put in your remark, the stronger your expression is. Eg. 1, you just scored a pentakill in some MOBA game. Your teammates could type 666 to show admiration. Eg. 2, you try to live broadcast yourself pulling off a kickass stunt and you landed on the ground face-first. Your viewers could comment 666 to mock your epic failure.