This is content on your website which has no links to it, and therefore hard to find by both search engines and those searching for your particular topic/keyword. The best way to get your content shown to the right people is to make sure that you have internal links (contextual links), external links and relevant links from other websites to your content. It is essential that you do not stuff your content with links that have no relevance to the topic. You should only link to other content which would enrich the information you are sharing.