please explain the quote :)

Something takes over in Chapter 17, shocking even Jerry. Something inside him does not allow him to accept the chocolate, even though he was planning on it and even looking forward to it. Cormier tips off the reader to the fact that this is perhaps the single most important moment in the book, by writing, "

This is the moment when everything changes. Jerry decides not to accept the chocolate. This will upend both Jerry's life and the underground society to which this school is based around, hence the quote:

Cities fell. Earth opened. Planets tilted. Stars plummeted. And the awful silence. This is a moment in which Jerry exercises his own decisions: he defies expectation, he defies Brother Leon and he defies The Vigils.