Halftone / Gradient Effect
A gradient is, essentially, a fade from one color to another.
It can be from black to white, grey to darker grey, red to pink etc.
It's all about how many dots of ink are printed in an area. In a halftone, the continuous tones of the picture being reproduced are broken into a series of equally spaced dots of varying size, printed with only one color of ink. The outcome exploits an optical illusion: the tiny halftone dots are blended into smooth tones by the human eye