Density is represented with the  icon.

Density is the newest Mark type, introduced in Tableau 2018.3. It fills a gap: to show the density of Marks. The superposition of multiple Marks determines the color intensity. It's a very simple Mark type; you can use it in various cases as long as you have many Marks overlapping. 

In the following example, you can see where the concentration of customers is by sales and quantity:

Now that you know how and when to use the 12 different Mark types, it's time to learn how to use another critical part of the Marks shelf: Mark properties.