A page or a post of your site will have a link. Place that link in other websites (it may be social site, or forum, etc). The link of your site placed anywhere in this condition is called backlink.
In search engine optimization (SEO) terminology a backlink is a hyperlink that links from a Web page, back to your own Web page or Web site. Also called an Inbound Link (IBL) these links are important in determining the popularity (or importance) of your Web site.
Backlink refers the number of hyperlinks that may direct more and more traffic to your website. These backlinks help in SEO, by directing traffic to your website thus improving the domain authority of your website and ranking on the SERP.