For five lines, the poem seems to be describing someone else. A girl, young and bright and hopeful, is held up like a portrait. Then comes the final line, set apart and signed with a tilde: "Who I used to be." The speaker has been looking at herself all along. That small reveal turns a simple description into a poem about time, and about the odd distance a person can feel from their own younger self.
The construction is plain, which is what makes it effective. Every line begins with the same words, "A girl," so the poem works like a chant or a litany, each repetition adding one more detail to the picture. There are also no conventional verbs in the main body. Nothing happens. The girl does not run, laugh or learn anything. She is simply listed, as in a photograph or a caption, frozen at one moment. The only verb in the poem is the one in the last line, "used to be," and it arrives in the past tense, as if time had been waiting to start moving again. The stillness of the first five lines is there so that the sixth can break it.
The lines also grow as they go. Counted by syllables (depending on how you say "naive"), they run roughly four, five, six, six and seven, so the poem swells gently toward its last description. The adjectives follow a similar path. "Young" and "naive" are facts about age and experience. "Full of life" is warmer and more general. "Made up of light" is outright metaphor, and "nothing but hope" ends on an abstraction. The girl becomes less a person and more a glow. Though there is no end rhyme, "life" and "light" sit side by side and echo each other in their opening l and long i sounds, which binds the two most radiant lines together. "Made up of light" is also a stronger phrase than "lit by light" or "surrounded by light" would have been. It says she is built from it, weightless and without shadow.
Two phrases cut against all this brightness. "Naive" is a word an adult uses. Children rarely call themselves naive, so the word quietly reveals the older voice behind the poem, one that can now see what the girl could not. Judging by the text alone, it carries a touch of tenderness and a touch of judgment at once. Then there is "nothing but hope." Read one way, it is praise, because hope is the best thing to be made of. Read another way, "nothing but" suggests that hope was all she had, with no experience or protection behind it. The poem never says what became of that hope, and that restraint matters. The speaker could be grieving, or fond, or simply amazed at how far she has come, and the reader has to decide. The poem gives no cause for the distance, only the distance itself.
The indefinite article also does quiet work. "A girl" is how you would describe a stranger glimpsed in a crowd or an old photograph. Only in the last line does the more intimate "I" appear, the poem's single personal pronoun, and it appears in the past tense. Saying "a girl" five times works as a kind of avoidance, as though the speaker needs the distance of a third-person description before she can claim the girl as her own. The tilde, like a handwritten sign-off or a caption added to a picture, sets that claim apart in a different voice, quieter and more personal than the lines above it.
The poem is significant because it does so much with so little. In about thirty words it describes an age and implies a loss, and it uses its one structural surprise to change how the earlier lines read. Anyone who goes back to the start after reaching the end will find "so young" and "so naive" sounding wistful where they first sounded descriptive. Almost everyone has looked at an earlier version of themselves and felt tenderness, envy or distance. This poem captures that feeling without explaining it, and leaves the reader holding the image of a girl made of light.