In the CMYK model:
C stands for Cyan.
M stands for Magenta.
Y stands for Yellow.
K stands for Black.
Printing processes typically use Cyan, Magenta, Yellow, and Black ink on white paper to reproduce colors. Unlike RGB color, which is ‘additive’ because light is added by the amount of color in each of the components, CMYK is ‘subtractive’ because light is subtracted from the bright white background whenever color is applied. This means that higher values for component colors create darker overall colors.
In theory, 100% of each of Cyan, Magenta, and Yellow should produce black, but in reality a pure black is not usually obtained. This is compensated for by introducing the K (black) component.
CMYK, like RGB, is a device dependent color model.