This feature deals with PostScript Level 2 or later printers, it instructs Photoshop to antialias the printed appearance of a low-resolution image. Halftoning is a process of organizing printer pixels into spots of halftone cells. Depending on how close the colored pixels are on a white background determines how light or how dark a color appears. These cells are so tiny that the human eye on first glance cannot delineate between the color and background. If you looked very closely or used a magnifying glass you would see what I mean.
Large cells appear to be dark shades while small cells appear to be lighter shades.
The cell size is measured in pixels and the maximum size is a function of the number of cells in an inch called screen frequency. Now if you turn off all pixels in a cell, it appears white while if you have them turned on, you'll get solid ink. Depending on the number of pixels you have on determines what shades you'll get. You can turn 0-25 pixels on and the printer will create up to 26 shades. You can change angle, size, and shape of individual halftone cells.