Oh, man. This one's tough. We don't have the faintest clue, actually.
Ahh, we jest. The title is, well, the protagonist's name. We see the world through Annie John's eyes and the book is titled Annie John.
But that's not all there is to this novel. The title Annie John, like the novel Annie John, is deceptively simple. It's not just about being Annie John; it's about becoming Annie John. Her name isn't mentioned for the first few chapters, and it's only in the last chapter of the novel that she speaks her name out loud. She (quite literally) is not named as Annie John until she starts coming into her own as Annie John. This is a novel about growing into an identity rather than being born into one.
Dagnabbit, Jamaica Kinkaid. Why do you have to be so subtly brilliant all the time?