Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. First the substance absorbs the enrgy, then it emits light. It is a form of luminescence.
In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation.
Fluorescence has many practical applications in mineralogy, gemology, chemical sensors (fluorescence spectroscopy), fluorescent labelling, dyes, biological detectors, and, most commonly, fluorescent lamps.
Fluorescence in the life sciences is a non-destructive way of tracking biological molecules. A protein or other component can be labelled with a fluorescent dye. This is often used in biological applications.