Manuscripts are sent out to reviewers who are asked to score the document.
Three factors influence the scoring:
The three scores get aggregated to a total score between 0-30.
The average of all total scores is the rating of the articles that is displayed. At least 2 total scores from 2 different reviewers are collected and averaged per document.
The reviewers who score the manuscripts don't know the author names. See: What is triple blindness?