A deep-link is a link from an external site to one of the pages deeper within your website i.e. not the homepage. The more links you have to different pages in your site, the more likely the search engines are to view your content as relevant and valuable. If all the links point to your homepage this may suggest a shallow site or that some level of automated linking has taken place which the search engines don’t like. It is also a good idea to deep-link internally i.e. from your homepage to a more buried page using your main keywords to reinforce importance and theme.