Backlinks are one of the most important factors for SEO(Search Engine Optimization). The number of backlinks indicates the popularity or importance of a website or a page. This is used by Google pagerank(basically google pagerank is a score -that shows the quality of website or webpage- for websites and webpages between 0 and 10. 10 is the best value.) which show algorithm for calculation of pagerank. For example facebook, twitter have lots of backlinks from many different websites in the world so they have a good pagerank value.
I have to say that, getting backlink is not an easy task.
And getting quality, high PR dofollow backlink is very hard. First of all you should create unique and quality content that when people see your content they want to share it. But the problem is if you have a new good site and content, how people get in your site without you are not in top search results. If you are not in top 10 search results this is not very easy. So to get in top 10 results, quality backlink is one of the most important factor. So you should introduce your site and your pages in internet world. Following the methods that show how you can introduce your content to the internet world.