A review snippet is a short excerpt of a review or a rating from a review website, usually an average of the combined rating scores from many reviewers.
When Google finds valid reviews or ratings markup, we may show a rich snippet that includes stars and other summary info from reviews or ratings.
In addition to the text of the review, a rating is an evaluation described on a numeric scale (such as 1 to 5). Review snippets may appear in rich results or Google Knowledge Panels.
Source: Review snippet.