IT'S EASY
When it comes to digital halftoning, the earliest and most basic form is white noise dithering, where each pixel of the image is compared against a random number. (I'm using the just green value of each pixel for comparison since I've found that it looks the best).
The randomness algorithm is uniformly spread such that the average value is 0.5 (scale of 0 to 1) to ensure that the image's pixels, while comparing against different values, will overall be compared against an average luminance of 0.5.
IT'S UGLY
White noise dithering, while a classic form, is generally considered a bad idea because the noise results in a messy-looking image where a lot of the detail gets lost.
In particular, the contrast between tones gets lost because there's no consistency or pattern behind what tones the pixels are compared against. This regresses all tones towards the mean of medium gray.