What do Schema tags do?

Schema tags are additional information inserted into HTML code that helps search engines better understand the meaning and relationships behind entities mentioned in your content, and can help encourage search engines to serve rich results in SERPS.

Users do not see schema tags. From a user perspective, schema tags are essentially private messages from a webpage to search engine crawlers.

Schema.org (often called simply schema) is a semantic vocabulary of structured data tags (or microdata) that you can add to your HTML. Structured data is a standardized format for providing information about a page and classifying the page content. For example, on a recipe page, the ingredients list, cooking time and temperature, the calories, etc. Schema.org is the result of collaboration between Google, Bing, Yandex, and Yahoo!

A common example of rich snippets produced by tags is the users rating stars for product pages.