Fluorescence is very useful for staining specific molecules within your sample and is the basis of much of what is done in the facility. The core principle of fluorescence is that the fluorophore is excited with one wavelength of light and light of a different wavelength is emitted. Common fluorophores are chemical fluorophores bound to an antibody, or fluorescent proteins. By forming an image from the fluorescence emission you learn where your protein of interest is located.

duke lmcf fluorescence principles jablonksi diagram
Here is what happens with a green fluorophore:
The fluorophore absorbs a blue excitation photon and an electron changes from the ground state to an excited state.

The electron then decays back from the excited state to the ground state and releases a green photon. The emission photon is always of lower energy (longer wavelength) than the excitation photon because some energy is lost in the process.
duke lmcf fluorescence principles spectra green red
Every fluorophore has characteristic wavelengths for excitation and emission and it often useful to view these on a graph like this.

This show a green and red fluorophore, because the spectra are relatively well separated it is possible to image both colours in one sample.