CMYK is a practical application of the CMY colour model in which black is used alongside the three primary colours (cyan, magenta and yellow) to enable digital printers to produce darker and denser tones.
CMYK refers to the four inks or inked plates used in some colour printing. The four inks are cyan, magenta, yellow, and ‘key’ (black).
The CMYK model works by overlaying colours that partially or entirely mask the background colour (usually white). The ink reduces the light that would otherwise be reflected.
Remember that when an observer looks at an image printed using CMYK inks, they are looking at the light that has first fallen on and then been reflected off the surface they have been applied to.