UV reactive clothing glows because it contains fluorescent molecules that absorb ultraviolet light and re-emit it as...
UV reactive clothing glows because it contains fluorescent molecules that absorb ultraviolet light and re-emit it as visible light your eyes can see. That is the whole mechanism. If a fabric holds those molecules, it glows. If it does not, it disappears into the dark, sometimes dramatically. Knowing which is which saves you from showing up to a blacklight party in a neon shirt that does absolutely nothing.