To reproduce full-color photographic images, typical printing presses use 4 colors of ink. The four inks are placed on the paper in layers of dots that combine to create the illusion of many more colors. CMYK refers to the 4 ink colors used by the printing press — the the subtrative primaries plus black.
C is cyan (a blue-green color),
M is magneta (a reddish pink color),
K is a pure black ink, the key plate or keyline color.
CMYK is not the only full color printing process but is the most common. The CMYK model works by partially or entirely masking colors on a lighter, usually white, background. The ink reduces the light that would otherwise be reflected. Such a model is called subtractive because inks “subtract” brightness from white.