First of all a backlink is a link that is placed on any other website on the Internet that links back to your website (homepage or inner pages).
Writing quality content can allow your site to naturally gain backlinks from other sites. If you write quality, relevant content and research your targeted keywords you can then have similar sites, with similar keyword strategies, linking to your articles because it is relevant, quality information. Also, when writing your articles you can incorporate outbound links to other relevant websites. Once they see you already linking to their site, then can then naturally link back to your site.