Cyan, Magenta, and Yellow are the three subtractive primary colours in colour reproduction.
Yellow represents the two-thirds of the visible spectrum which remains when all blue light has been subtracted. Likewise Magenta is white light with green subtracted; and Cyan with red subtracted.
Stacking filters of cyan, magenta, and yellow in front of white light will (theoretically) pass no light, resulting in black.
In reproducing images on paper, it is common to include a separate black ink component. To avoid confusion with B meaning "blue" (in RGB), this is abbreviated K, thus CMYK. Digital camera and computer-screen images are natively RGB, and must be converted to CMYK for printing, typically by the driver software.