Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
Like a wild animal, the truth is too powerful to remain caged. —From the Candor faction manifesto
Veronica Roth is cheating a little bit here, taking the epigraph from a fictional text within her own universe. But it's still worth looking at, as it sets the tone for the whole book.
Insurgent is about Tris's search for the truth—or truths, really, since she wants to know 1) the real reason her parents died, 2) what it means to be insurgent, 3) what Jeanine is really up to, and 4) what Chicken McNuggets are actually made of.
Unfortunately, none of these questions are actually answered in the end. The end just raises more questions. Maybe the truth is more like a wild animal that realizes it actually likes it in the zoo and wants to stay in the cage, thank you very much. The harder Tris tries to coax it out, the more the truth tries to stay hidden.