The midpoint is the point of no return in the story, where the character undergoes a crisis, enlightenment cracks the shutters. The main character begins to wake up to the the qualities necessary to reverse the misfortune or pursue the action of the book through to a resolution she or he can live with and accept.
This is the point in the story at which the main character is most challenged and at their lowest. It speaks volumes about your true intentions for the book.
You may not be clear about these at first draft - I am usually a little in the dark - but locating the belly button will help you sort out 'what the hell is this book really about.' The idea is developed in John Yorke's book Into The Woods as a convention axiomatic to the development of screenplays.
'The midpoint in our change paradigm corresponds to the moment of Vogler's 'supreme ordeal'... the...