In this investigation, students add a phosphor to soap mix to make some fluorescent bubbles.
A black light gives off harmless, highly energetic, ultraviolet (UV) light that is invisible to humans. Certain fluorescent substances absorb ultraviolet light and re-emit it at a different wavelength, making the light visible and the material appear to glow.
What you see glowing under a black light are phosphors. A phosphor is any substance that emits visible light in response to some sort of radiation. Here, light is the form of radiation.
UV light makes certain objects glow because the objects contain "phosphors" which absorb the energy from UV light then release it again as visible light.