Drupal has a very flexible taxonomy system for tagging and categorization content (and users, and anything else). This flexibility is one of its interesting differentiating features — you can create as many dimensions of categorization and tagging as you like, which is very attractive for librarians and other enthusiasts of organization. To keep that flexibility from totally bogging down the system, Drupal maintains an index table that maps those terms onto the pieces of content to which they're attached.