In the last two paragraphs of the book, Eldric says he loves Briony and she believes him—and when she does, she finds she has renewed faith in herself. For the first time in forever, she believes that she is lovable and to hope in a brighter future. Yay. To say the least, this has been a long time coming.
When she questions how a word can build a whole world, Briony is wondering at how Eldric's admission of love can open up her heart and belief in the future so much and so quickly. Is it really the word that has changed things for her, or the feeling of having someone else care for and accept her as she is? Considering the years of damager her stepmother has done to her psychologically, will that word alone be enough to change Briony's negative thoughts? As the book ends, she's committed to trying.