Star rating is the public score out of five that a client gives a freelancer or agency when a contract ends, shown on the work history for that contract and averaged across all rated contracts at the top of the profile.
When a contract closes, the client rates several aspects of the work, such as skills, quality, communication and adherence to schedule, and Upwork combines them into one rating out of five for that contract. The written comment and the rating together are the public feedback on the contract. The profile's headline rating is the average of those contract ratings, and freelancers rate clients in the same way, which is why a client's own profile also shows stars.
The rating is often mistaken for the Job Success Score. They are different measures. The star rating is a simple public average in which every contract counts equally. The JSS is Upwork's own calculation, weighted by contract value and length, built partly from private feedback the freelancer never sees, and sensitive to contracts that end badly or with no feedback at all. That is why a profile can show 4.9 stars and a JSS in the 80s at the same time.
Because most clients rate generously, the spread of star ratings on Upwork is narrow and the number of reviews behind the average matters as much as the average itself.
How Upwork Insights uses it
Upwork shows a freelancer's Job Success Score only to signed-in visitors, so the freelancer ranking of 11,749 profiles with $10,000 or more in public earnings carries the public star rating and the number of reviews behind it instead. In the freelancer Index Score the quality component uses the JSS where Upwork publishes it and falls back to the star rating otherwise, as set out in the methodology. Agency pages show the JSS, so for agencies the index reads that rather than stars.