For each assignment, every student receives a "Feedback score" between 0% and 100%. This score is an indication of how good the student is at giving feedback to other students.

Together with the submission score, the feedback score is counted in the total score for a student.
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The feedback score for a reviewer is based on the feedback reactions they receive (see here what a feedback reaction is). When students receive a review, they are asked to react to it using a scale ranging from "Not useful at all" to "Extremely useful".

These ratings each correspond to a score between 0% and 100%. The feedback score is the average of the reaction scores.
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Imagine that a student has completed 4 of 5 reviews and received feedback reactions to 3 of those 4 reviews. Then we consider the reactions to each of the reviews:

Review 1                 4/5 (75%)
Review 2:               No reaction received for this review
Review 3:               1/5 (0%)
Review 4:               5/5 (100%)
Review 5:               Did not complete this review (0%)
Feedback score:   (75% + 0% + 100% + 0%) / 4 = 43,75%

Note here, that since no reaction was given to the second review, we do not take this one into account for the feedback score. This also means, that if a reviewer does not receive any feedback reactions, then they don't receive any feedback score.