Halftones are a mathematical dot pattern that represents the grayscale value of a color and are created based on a grid. Printers can rotate the grid to change the angle, lines per inch, and more. This dot pattern can be so tiny that it appears as a regular image, but can be screen printed easily.
The halftone process translates the different grayscale tones of a photograph into dots of various sizes. Halftones interpret grayscale values based on a predefined grid or pattern using Lines Per Inch (LPI) and Angle, making them useful for screen printing.
To learn more about halftones, check out this article.