User generated content is any multimedia content on a webpage that is generated by the site users, rather than the site administrators. Examples are blog comments or forums.
Getting the users to create content is a fantastic resource as you can create much more content for your users and for the search bots to crawl. Sites like YouTube, Quora and even Facebook rely on their users to create content for them and have done pretty well off the back of it. While you might not be the next Mark Zuckerburg, ensuring that your blog has a forum is a good way to create more content and encourage users to engage in a more meaningful way. Be careful and make sure you moderate the comments well.