All materials either reflect or absorb light at distinct
wavelengths or
colors. Any absorbed light turns into energy and this energy is then
emitted back out of the object, usually as
infrared because the
radiation is
produced by chaotic motion. However, some materials produce some of the radiation less randomly and therefore some of the wavelengths of the light they emit are higher and therefore
might be visible. When light of one wavelength is absorbed and its
energy is then emitted at a lower but still visible wavelength, the material doing the absorbing and emitting has the property of
fluorescence.
A
black light that shines its
invisible high-energy light on something fluorescent gives that thing the energy it needs to emit visible light.