In this poem, the speaker recalls a trip on a stagecoach. He explains how a young girl catches his attention. He wonders why she doesn't look outside and admire the scenery as a child typically would. She informs him that she is blind, and he finds this incredibly mournful. The girl's mother then explains how the girl simply lost her sight in the middle of the day.
Depending upon your interpretation, the last two lines could apply to the child, the mother, or both. The sun shines on the mother as she relates this tragic tale, but her eyes receive no light. This would be metaphoric, meaning that she is so sad that her eyes cannot take in light, warmth, or solace. In the other case, the meaning is literal and/ or metaphorical. The sun shines on the girl as she speaks but her eyes receive no light because she is blind and cannot see the light. Metaphorically speaking, like the mother, the young girl gets no light, comfort, or warmth from the outside world. She might get comfort from her mother's words, embraces, and so on, but she gets nothing visually.
In this poem, "light" is the key metaphor. Light typically symbolizes ideas like goodness, warmth, truth, and love. Therefore, the girl's blindness represents a lack of light. In this case, she experiences a lack of warmth and comfort that visual experience might provide.