Title text: You'd be moved up from 49 of ~7 billion to 31 of ~7 billion.
"I'd tell you, but then I'd have to kill you" is a flippant response to a question that's been around at least since the movie Top Gun, and has entered regular use in the English speaking world, even among people who don't know its origin.
The Cueball uses the line here, but the joke is that he actually is planning to kill the the other one, and if he answered the question he'd have to kill him even sooner.
According to the title text, he'd go from #49 on his hit list which apparently includes the entire world population to #31.
The title 'Joshing' refers to the colloquial American verb 'to josh', meaning to joke with.
- Cueball: So, is the new project going forward?
- Friend: I could tell you, but then I'd have to kill you!
- [The men laugh cautiously.]
- [The men resume conversation.]
- Friend: I mean, kill you even sooner.