Schema.org markup (often just called Schema) are tags containing tiny bits of information (microdata) that can be added to HTML to change (and possibly improve) the way search engines read and represent your page in SERPs. For example, seeing a 5-star review in an organic listing is a result of Schema.
Schema was created through collaboration between Google, Bing, Yandex, and Yahoo!
Officially, there is no conclusive evidence that Schema improves rankings. However, good schema that web visitors may find useful could lead to a click. So, in this regard, good schema markup could lead to a better click through rate (CTR) but does CTR increase SEO performance? Not according to what was said last year by Gary Illyes of Google who serves as the head webmaster trends analyst:
“Dwell time, CTR, whatever Fishkin’s new theory (Moz) is, those are generally made up crap. Search is much more simple than people think,” Gary said.
Any time that I personally used schema.org markup (adding reviews in my listings, recipes) I never saw a change in rankings from it solely. For something like recipes, or other pieces of standalone content, schema.org markup can make it easier for Google Search and other major search engines to index and organize properly but that doesn’t equate to good rankings. That only equates to search engines being able to communicate with your content. At the same time, with something like recipes, schema.org markup is practically a requirement since so much detailed and precise information can go into a recipe.
In conclusion, there really isn’t anything that structured data is going to do for a website in terms of SEO. Schema can, however, speed up the search engine indexing process and possibly increase CTR by revealing details that would have been otherwise hidden from a search results page.