On a printing press each color of ink used in a document is printed one at a time. Each printing plate used on the press is made up only of components of the page that have that one color. Separations are artwork split into component plates of cyan, magenta, yellow, and black in preparation for process printing (CMYK) or into the required number of plates for spot color printing.
Each separation prints a single process or spot color. Digital files can be composite separations (all information in one file) or preseparated (each color on its own page).
During prepress you should print separations to your desktop printer to insure they will separate properly once sent to your commercial printer.