It is called monochromatic light source to a source that emits light of a single color. Natural light or white light is composed of several colors; these are the colors that can be seen in the rainbow. The decomposition of natural light into it's colors is called dispersion of light. This is a phenomenon that can be observed when a light beam is refracted continuously in two surfaces, such as the surfaces of a prism.
According to the quantum theory, the natural light is formed by a photons flux of different frequencies and wavelengths. In the case of a monochromatic light, the beam will be formed by a flux of photons of frequencies and wavelengths equals.
An example of monochromatic light source is a "LASER source", which emit a specific color according to the substance "excited", for the emission of the monochromatic light.