This visualization is designed to allow you to explore how users got to your website, and who they are. There are two ways you can do this.
The Basic View shown intially groups users by the channel they used to get to your website, with larger bubbles representing more users. Clicking on a bubble will allow you zoom in a level to look at that group of users more closely.
Clicking the Advanced View tab will change the chart to a zoomable treemap. In this chart, all the options are customizable, allowing you to select all dimensions including what metrics are represented by the color and size of each area. You are also able to define which dimensions should be included, and in which order, changing the order of the levels as you zoom in through the treemap.
Size and Color