The novel starts with a request from the narrator: "Call me Ishmael."
No, wait. It's Jonah. The narrator wants you to call him Jonah. Where'd we get Ishmael?
He wants to be called Jonah because "[c]onveyances and motives, both conventional and bizarre" have led him to be at fated places at the right time (1.2).
But, you know what? His real name is John, so we're going to be calling him John.
(Psst: This has a lot to do with the Biblical tale of Jonah. We can't get too deep into it here and now, but swing on over to our "Characters" section for a little exploration and explanation on how the two stories relate.)
Anyway, when John was younger, he set about writing a book called The Day the World Ended. It was to be a factual account of the atomic bombing of Hiroshima, Japan.
Oh, and John is a Bokononist now, meaning he believes "humanity is organized into teams" that do God's will. These teams are called karass.
John meets his particular karass through a kan-kan (not can-can), or instrument of fate.
And, yes, these words are supposed to be funny and not—repeat not—to be taken too seriously.
That instrument was his unfinished book, The Day the World Ended, but seeing as John Hersey already wrote the definitive book on the subject, it's probably best he didn't finish.