The report shows the top defects affecting the most test cases created over a period of time for the current Project. It displays the defects which are linked maximum times with test cases. The defect type should be "Bug".
The Usages count on the report indicates the number of times that particular defect is linked with test cases.
Filter Report:
Click on the Filter icon 
on the individual chart to generate the report as per requirement by applying multiple filters.
