They are: threshold, ordered, and dithered.
Threshold basically just converts an image to 2 colors, black and white.
Ordered performs dithering (approximated shading) using an ordered pattern.
Dithered uses an error diffusion dithering to approximate shading by distributing the conversion error of each pixel to its neighbors. This tends to work best for photos.
The LightBurn documentation has a really good explanation of these.