Fluorescence is the phenomenon whereby molecules that have absorbed near ultra-violet and visible light emit light at a longer wavelength (usually in the visible spectrum).
In experimental practice, this is of very good use for imaging: you can shine light that causes fluorescence in the molecule you want to image. Then, if your image capturing device only works for a certain wavelength range (in this case, the range of the emitted light from fluorescence), then your imaging device will not "see" the original light that was shining on the sample.
 B.H. Brandsden, and C.J. Joachain, "Physics of Atoms and Molecules," Second Edition, Pearson Education Limited (2003).
 A. Kuzmich et al. http://www.physics.gatech.edu/qoptics/currentresearch/quantumrepeaters.html