This is referring to the start of Chapter XIII where Tom and Joe meet (and later get Huck) and decide to run away because of how badly they are being treated.
We don't exactly get told how Tom convinces Joe. All were are actually told is this:
after listening to Tom, he conceded that there were some conspicuous advantages about a life of crime, and so he consented to be a pirate.
I think if you look a bit before the quote and a bit farther down you can see what the advantages might have been. Before the quote, Joe wants to be a hermit and get cold and hungry and die. That's probably not that much fun.
And then afterwards, you see all the cloak and dagger stuff they're doing? They have passwords (Blood!) and code names and stuff. It's perfect guy stuff, right?
So I figure Tom told him that he could go die cold and hungry or he could have fun with passwords and nicknames.