The Semantic Web is the concept that you can add machine readable meaning to you website to make it easier for computers as well as humans to understand what is going on with your content. If you look at this page you will find it easy to tell which part of the page is blog content, where my name is etc., but for a machine this is not so easy, they will only see something like “bla bla bla TITLE bla bla bla” and often will be limited to trying to find content based on keywords. The whole Semantic Web idea is the subject of major research efforts and much debate over how far it can go, but the good news for Drupal site administrators and readers of their websites is that Drupal 7 will be offering support for some of this technology out of the box.