Google's SERPs are populated by what are commonly called snippets, which attach short text descriptions and links to cached versions to the prominently displayed links pointing to the most relevant URLs Google can pull up for a given search query. Snippets display the text contained within a meta description tag inside the HTML syntax, but if the website does not include these tags, Google automatically pulls a span of text from the content of the page itself to fill this text out. The meta tag is only one method web designers have of remotely influencing the snippets representing their websites on relevant SERPs, however.
Schema markup is what is ultimately responsible for various websites on Google being represented by uniquely structured and expanded snippets for the benefit of potential customers. For example, a website that represents a company or group that operates on a clearly communicated schedule can make use of schema markup to display on its representative SERP a timer counting down to a particularly important upcoming event. Similarly, many products that are sold on online storefronts will display five-star ratings to tell users how favorably the customers for that product on that site are viewing it. This information would be pulled from the product page itself and routinely updated within the snippet so that Google users searching for the product will not have to directly open up the web page to get the basic idea behind how highly customers are reviewing it on the website.
While schema markup can be implemented into a website by being inserted into the HTML syntax, it comes in three different formats, only one of which is seen as friendly enough to web designers that they can implement it personally. This take on schema markup is categorized as JSON-LD, and website owners can download templates for this markup from Google and make changes to them before running them through the Structured Data Testing Tool that Google also provides. Another Google utility named the Tag Manager can then be used to insert the markup syntax into the website's HTML files. For more information click here https://www.reddit.com/r/SEO/comments/cgq985/cansomeonehelpexplainschematome/.