The Improvable Questions widget displays the question(s) that have lost the most amount of points over the time period selected.
The "Columns" widget settings allows you to display the data in four different ways.
Breakdown setting displays details for the selected questions.
Points lost displays the amount of points lost for the selected questions.
Potential Impact is the amount that the overall score would have increased if the question was answered perfectly.
Potential Evaluation Score is the overall score that the evaluation would have if the question was answered perfectly.
Remember to check a different help article detailing most of other settings of this widget.