Review (schema.org)
The schema.org type Review allows individual reviews to be marked up in structured form and can lead Google to display them as a rich snippet with a star rating, author name and review text.
In practice
The precondition is that the reviews are genuinely visible on the page and were not invented, bought or generated automatically, because Google issues manual actions against fake markup of this kind, which costs the page its rich result eligibility. Review can be combined with Product or LocalBusiness, and with AggregateRating for an overall picture of several reviews. One restriction that Google states explicitly is decisive here: where the business being reviewed controls the reviews about itself, pages carrying LocalBusiness or Organization markup are ineligible for the star display. Testimonials collected on your own website therefore produce no stars in the search results even with technically flawless markup; that requires independent review platforms.