This field is responsible for aggregating created defects in Test Cases on given Test Plan. All defects are displaying in Defects from Test Cases panel in the Test Plan. You can decide which defects you want to show based on the defects Resolution and Test Plan Iterations. The configuration regarding the Defects from Test Cases panel you can find at TestFLO Settings in the Defects section.
The TestFLO - TestPlan Defects is locked by default and is not added to any issue screen. If you want, you can add it to screen to display the list of Defects issue keys. Adding this field to issue screen does not affect the display of the Defects from Test Cases panel.
The panel and defects will be displayed with a delay from the moment where the defect was created or linked in the Test Case. This is due to the data search in Test Case executions.