Fluorescent chemicals are those that are capable of accepting light of one wavelength and converting that light so that it is given off at a different wavelength of lower energy.  Many fluorescent chemicals are capable of accepting light in wavelengths that we cannot see with our eyes, and giving off that light in wavelengths that we can see with our eyes.  That is why florescence highlighter ink appears to be brighter than anything else on a page.

In life science fluorescent chemicals are attached to proteins, nucleic acids, drug compounds, and other kinds of biomolecules, to give off a bright signal that allows us to keep track of, and measure the presence of these molecules.