The above quote indicates the very powerful magnetism that Antonia exerts on Jim, the narrator of this novel, throughout the story. Remember the title of this book and how it represents the way that it is in so many ways a pageant to Antonia and to the frontier spirit of optimism and hope that she represents for the narrator. The quote above indicates the way that Antonia as a character enlarges the narrator's understanding of life through her stubborn, unyielding nature and her determination to accept life only on her own terms. Note what Jim says about her just after this quote:
All the strong things of her heart came out in her body.... She was a rich mine of life, like the founders of early races.
The powerful metaphor, comparing her to a "rich mine of life" indicates the way that her character captures something of the unquenchable human spirit that continues to defiantly survive and to continue living whatever the odds, not letting the vicissitudes of life shape them. It is this characteristic of Antonia that inspires the narrator so greatly and "fires" his "imagination." In Antonia, he sees a glimpse of the true glory of being a human being, and is challenged and awed by this sight.