The speaker wonders why girls who seem "broken," like cracked glass, often come across as kinder and brighter than those who look flawless, like unmarked porcelain. The answer arrives as an image from science: a prism. A damaged surface, with its angles and distortions, can split a single beam of light into many colours. The speaker suggests that these girls have learnt how to do exactly that, turning their rough edges into something that sparkles.
The tone is gentle and admiring. There's curiosity at the start, then a growing certainty, and the speaker clearly finds these girls beautiful, not pitiable. The short final line, set apart from the rest, lands like a lesson: "light is what you make of it." Pain doesn't have to be hidden or fixed. How a person handles it, and the angle they hold it at, can make it a source of colour. The mood is hopeful and quietly encouraging.