A fluorochrome is a dye which stains the cells. It fluoresces if illuminated with a particular laser wavelength. Shown here is illumination (excitation) with blue light. The dye fluoresces (emission) in green light
The big trick in multifluorescence staining is to select particular fluorochromes whose spectra do not overlap. Modern confocal microscopes have up to three different color channels, good for the separate detection of three fluorochromes.