A fluorescence microscope makes selected structures stand out by using molecules that glow after absorbing light. These fluorophores may be dyes or fluorescent proteins attached to a target in the sample. Ordinary illumination alone would show a different picture.
Excitation and emission
The microscope sends an appropriate excitation wavelength toward the specimen. A fluorophore absorbs some of that energy and emits light, usually at a longer wavelength. Optical filters keep much of the excitation light away from the camera or eye, helping the much weaker fluorescence become visible. Different labels can sometimes be distinguished by their colors.
What a bright spot really means
Brightness is evidence of emitted light from a label, not a direct photograph of every molecule. Poor labeling, overlapping spectra, background glow and fading under illumination can complicate interpretation. The method is powerful because it can highlight where a chosen structure is, but the conclusion depends on how specifically the label binds.
Related: how a spectrometer separates light by wavelength.