What is screening in printing?

Look closely at any printed photograph and the dots are there. At normal viewing distance the eye blends them into smooth tone; under a loupe they resolve into a regular grid of varying sizes. That is conventional amplitude-modulated screening, and it is what most commercial printing uses.

The fineness of the grid is measured in lines per inch. A higher ruling gives finer detail and smoother gradients, but only if the substrate can hold it. Coated art board holds a fine screen well because the ink sits on the surface. Uncoated and kraft stock absorbs ink and lets it spread, so a screen that is too fine simply fills in and the shadows go solid black. Corrugated is coarser again. Choosing the ruling to suit the board is a prepress decision that visibly affects the result.

Frequency-modulated or stochastic screening takes a different approach, using dots of a fixed very small size distributed with varying density. It can hold more detail and avoids moiré patterning, at the cost of being more demanding on press.

For a buyer the practical point is that the same image will not look identical on coated and uncoated stock, and no amount of press skill will make it. If the image is critical, prove it on the actual substrate.