A backlink is simply a link that appears in web content on the Internet that leads back to another website. In this case, the link would lead back to your hubs here on HubPages.
Sometimes this occurs naturally when someone happens upon your hub, likes what they read and, if they have a website or blog, include a link in it that leads back to your hub.
But most Internet-savvy people don't leave this to chance; instead they create backlinks, for example, through social networking sites like Facebook, Twitter, and Google+. In this instance, you would create a post that includes a link that leads back to your hub. Then there are sites like Social Monkee and Amplify whose primary purpose is to let you create backlinks to your hub. If you have a blog, you can include a link in one of your blog posts that leads readers back to your hub.
Writers, bloggers, and website owners put a lot of time into creating backlinks because this is one of the things Google takes into consideration when it returns search results to people looking for information. They consider a site that has many links leading back to it from other websites as "authoritative" and therefore having the high quality information it wants to provide to search engine users. The goal is to get your hub to appear as close to the number one spot on the search engine results page as you can get it.
Hope this answers your question.