Printing colour built from dots of cyan, magenta, yellow and black ink, rather than a separately mixed ink for each colour.
Halftone dots of the four process inks overlap and sit side by side; the eye blends them. The achievable gamut depends on the inks, the paper and the press, which is why CMYK values need a profile to mean a specific colour.
CMYK is not one colour space. The same CMYK numbers print differently on coated and uncoated paper.
Most magazines and brochures are printed in process colour.
It explains why some screen colours, especially bright blues, greens and oranges, cannot be printed as they appear.