Simply summarized by urbandictionary.com "A sarcastic comment that is used when someone figures out something really obvious. An imitation of a game show host announcing that someone has given the correct answer and has won a prize that his assistant (Johnny) will specify on cue."
I would add situations in which someone is slow to understand a joke.
For example, A makes a joke. B laughs politely but doesn't really get it (understand it). 5-10 minutes later, B realizes the meaning of the joke and says, "oh, I get it. You meant blah blah blah." Then, A might say, Ding Ding, What do we have for em, Jhonny?
It can be pretty insulting to say this to someone. Also, it would sound pretty funny being said by a non-native speaker. I would surely think, "where did you learn that?" But, it can also be used for joking around with your friends.