The dot pitch of a color monitor measures the size of the tiny individual dots of phosphorescent material that coat the back side of the picture tube's face. The dot pitch helps determine how sharp the image looks, independent of the resolution (which is measured in pixels). A smaller dot pitch is better.
Here's the technical scoop: Each point of light on a color monitor is formed from a triad of three separate dots of phosphor: one that glows red, one green, and one blue (the color you finally see depends on how intensely each dot in the triad is excited by the picture tube's electronic beam). The dot pitch is the vertical distance between the centre of one dot and the next like-colored dot directly above or below it (the way the dots are arranged, pairs of like-colored dots are always two rows apart). The farther apart the centres of the dots are, the bigger the dots and the fuzzier the image. All other things being equal, a monitor with a smaller dot pitch is preferable to one with a larger dot pitch, though other factors are more important in determining image sharpness below a certain dot pitch threshold.
A dot pitch of .28 mm or smaller is ideal for 14- or 15-inch monitors; a dot pitch of .31mm or less for 17- to 20-inch monitors. Resolution, in pixels, is determined by the video circuitry in your computer. Depending on the resolution and on the dot pitch, a single pixel may occupy 4 to 16 separate phosphor triads.