The term stipple engraving is only half correct. Yes, the image is created by stippling, but stipple engraving is actually an etching process. The artist works on a metal plate that has been covered with an etching ground which is acid resistant. The artist then creates the image by making stipples (small holes) in the acid ground. The image is physically created by applying the acid over the punctured acid ground where it will bite the plate at each stipple hole. Stipple engraving is really not tonal in a physical sense because the dots are bitten at the same size, but a tonality of the image is created by the concentration of the dots in different areas.
Here we have a visual example of how the concentration of dots creates tonality. The small image to the left appears to be constructed of different shades of gray, the darkest at the center and getting lighter as we approach the white. This magnified image below reveals that the image is actually just black dots spaced differently. It is the same idea as crosshatching with lines--the close packing of fine black and white elements will visually blend to create a gray area.
Stipple engraving is an INTAGLIO PROCESS, and stipple prints will have a plate mark.