Review schema

Review schema comes in two related schema.org shapes. Review marks up a single review and needs author, an itemReviewed, and a reviewRating containing a ratingValue. AggregateRating summarizes many reviews and needs ratingValue plus at least one of ratingCount or reviewCount. Google supports it on types like Product, Recipe, Book, Course, Event, Movie, SoftwareApp, and Organization or LocalBusiness.

A minimal aggregate example:

{
  "@context": "https://schema.org",
  "@type": "Product",
  "name": "Trail Runner 2",
  "aggregateRating": {
    "@type": "AggregateRating",
    "ratingValue": "4.4",
    "reviewCount": "218"
  }
}

The self-serving rule

Since 2019 Google will not show stars when the entity being reviewed controls the reviews about itself. A LocalBusiness or Organization page reviewing its own business is not eligible for the star feature, whether the testimonials are hand-typed or pulled in through a third-party widget. Ratings must come directly from users, must be readily visible on the marked-up page, and cannot be scraped or aggregated from other sites. Use a dot for decimals (4.4, not 4,4), keep author under 100 characters, and mark up one specific item rather than a category or list.