A dot density theme is a feature style modifier that draws the fill pattern of a region using dots based on the numeric value of the theme's expression. Dot density themes use dots to represent the data value associated with a boundary or region. The total number of dots in a region represents that region's data value. For example, if you have 10,000 senior citizens in a county, and each dot represents 100 senior citizens, there would be 100 dots in the county boundary.
A dot density theme is useful for showing raw data where one dot represents a large number of something: population, number of fast food restaurants, number of distributors who carry a brand of soda, etc.
For example, if you have a table of population broken down into county boundaries, you could use a dot density theme to show the concentration of people in each county boundary. There are two properties you control for dot density themes. You can specify the value of one dot. For example, to represent 20,000 high school students in Rensselaer County, New York using a dot density theme, you can specify that one dot represents 200 students. When the county is drawn, the 100 dots would also get drawn inside the county.