Overall feedback
The overall feedback is some text that is shown to a student after they have completed an attempt at the quiz. The text that is shown can depend on the grade the student got.
For example, if you type "Well done" into the first feedback box, type 40% in the first grade boundary box, and type "Please study this week's work again" in the second feedback box, then students who score 40% or better will see the "Well done" message, and students who score less than 40% will see the other message.
The grade boundaries can be specified either as a percentage, for example "31.41%", or as a number, for example "7". If your quiz is out of 10 marks, a grade boundary of 7 means 7/10 or better.