As a transitive verb, the act of stippling involves covering an area with dots. What comes right to mind is a wildly time-consuming technique, done with a technical pen and ink (usually black), in which an image is drawn dot by dot by dot. (One may also stipple glass, an engraving plate, a quilt, or even an interior wall.)
The resulting image contains no lines. It is a collection of dots, strategically placed to suggest forms, shapes, contrast, and depth. It is left to the viewer's eye to complete the picture-a proposition which seldom fails.
Stippling is also the manual forerunner of Benday dots and halftones. (For you young'uns out there, these were graphic image tools employed before the advent of the computer pixel.)
Pointillism is a close relative of stippling, in which the artist, using brushes and different colors of paints, creates an entire composition out of dots.
As a noun in this instance, stippling is what one sees, and is the end result of someone using stippling as a verb.