In the color printing, ink of only 4 colors are used, which are cyan, magenta, yellow and key(black). When CMY “primaries” are combined at full strength, the resulting “secondary” mixtures are red, green, and blue. Mixing all three gives an imperfect black. And for each color, it has infinite shades and tones. (learn more here about the color basics, hues, tones, shades and tines).
The problem here is that how can we simulate the different shades, like pink color, of a color with the only the magenta color ?
In the 19th century, the graphic designers find that the discrete dots, varying either in size, in shaping or in space can used to generate a gradient like effect. And this reprographic technique is called halftone.
Check the picture below for a better and clear understanding:
Left: Halftone dots. Right: How the human eye would see this sort of arrangement from a sufficient distance.