Fluorescence is a mechanism whereby a molecule absorbs a photon, then emits a lower-frequency photon. If an absorbed photon results in a higher electron state of excitation, photons may be emitted in smaller photon-energy steps (using intervening states of excitation), or some kind of cooling may allow the state of excitation to be reduced (conduction, or interaction with other molecules) before a photon is emitted. If the molecule is ionized and later captures an electron, its path back to the ground state may offer small steps. A result is the emission of characteristic frequencies corresponding to electron transitions. In everyday fluorescent lights, an electrified gas tube that produces ultraviolet light is surrounded by a material that fluoresces as (lower-frequency) visible light.
The term fluorescence is used for the case where the material loses its excited state (emitting a photon) with a very short timescale, tiny fractions of a second, so when the EMR that is triggering it ceases, the fluorescence ceases immediately. When the material's state's timescale is longer, e.g., seconds, minutes or more (e.g., due to other quantum-mechanical rules making the state metastable), the phenomenon is termed phosphorescence.
Biofluorescence is this mechanism in operation within lifeforms, such as on planets. Fluorescence also occurs in gas (and plasma) such as clouds, atmospheres, and accretion disks.