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.
Dark is a way and light is a place,
Heaven that never was
Nor will be ever is always true
-- Dylan Thomas "Poem on His Birthday"
What's up with the epigraph?
Dylan Thomas also wrote "Do Not Go Gentle into that Good Night," which would also be a good choice for an epigraph, because Jeannette never goes gentle into her terrible childhood. She fights against it and triumphs.
This poem is another one about life and death, and dark and light. In her life, Jeannette has to travel through a lot of darkness to reach the light, or New York City, which has a lot of lights. But the key part of the epigraph is the second and third lines. A "heaven that never was" is a poetic way of describing the Glass Castle her dad always promised but never built. But the final part of this excerpt says that it "is always true." That clues us in to the fact that even though the Glass Castle was never built, it is true in Jeannette's memories. She's able to see through the darkness and understand the complex truth of her childhood.