The stacked bar chart is based on two different fields: series and category.
Th series are represented with colors that are stacked in each bar, and the bars themselves are the category field. Together, the report shows the number of each series within every category and the total category count.
The example above is showing priority as the category. Each bar represents a priority from lowest to highest and each color represents the number of issues from a specified source like email or phone with that priority. At a glance, you can see which priority you have the most of and in that priority category, which are your biggest sources of issues.
To generate the report, you have to specify a series field, in this case source, and a category field, in this case priority. You also have to give it a project key, display values, and specify the types of issues to include. There are some additional optional fields to enhance your report, like the ability to add custom JQL.
As with all standard reports, the expanded view offers you more detail. You can see the exact count from each source with a given priority and the links will take you to the issues in Jira.