I have heard about the concept Schema markup, how does it affect the ranking in search engine?

The concept of schema markup is to give your site a better search engine ranking but you are asking how it affects search engine ranking? Well, I don't really get it but what I can say is that schema markup helps return the search engine an informative results. Schema tells the search engine what your data means, not just what it says.

Schema Markup can help increase search engines’ understanding of your site’s content, as well as enhance search visibility via rich snippets, featured snippets and Knowledge Graph results. Structured data is a great way to create more search engine-friendly signals, which can indirectly impact search rankings.