Linking CiviCRM contact data with Drupal data has until now not been possible directly. For example, imagine you run a cycling club. You have some members who are interested in mountain bikes, some in road bikes, and some in BMX bikes. You want to create "category" pages on your website that link users and content together in a community. In other words, if you could tag contacts and content with "Mountain bike", you could create a page that displays the two together.
Before now, you would have to create a user account on your website for each contact and link them both using a tag. Now you can do it directly. This makes it possible to produce category pages that list linked contacts without the requirement for a user account.