The Improvable Sections widget displays which sections have lost the most points over the time period selected.
The "Columns" widget settings allows you to display the data in three different ways.
Points lost displays the amount of points lost for the selected sections.
Potential Impact is the amount that the overall score would have increased if the section was answered perfectly.
Potential Evaluation Score is the overall score that the evaluation would have if the section was answered perfectly.
Remember to check a different help article detailing most of other settings of this widget.