When electrons absorb light, they move to a higher energy level. The electrons lose this energy by falling back to their original energy levels. If the initial jump was more than just one energy level, the electrons may return to their original energy levels through a series of shorter falls. In certain conditions, the original jump may have been caused by ultraviolet light. The shorter falls may then occur in frequencies in the visible light range. This phenomenon of objects emitting light after having been exposed to other radiation is known as fluorescence.
