Review schema (Review markup)
The Schema.org Review and AggregateRating schemas mark up ratings and reviews for a product, service, book, or recipe to trigger star display in search results and increase click-through rates. Eligibility is limited to specific entity types defined by Google.
In practice
Google allows Review markup only on specific entity types: products, books, films, recipes, software, and local businesses. Marking up stars on a homepage or generic service page violates the guidelines and can trigger a Search Console warning. The displayable rating range is 1 to 5; providing a real reviewCount value avoids penalties for misleading data.