A block in Drupal is an area of the page above or below the main content or in a sidebar (though no pages at time of writing have a sidebar region enabled). A block may contain a menu, some content auto-generated by code, or some HTML. This could be used for adding an advertisement for example, by going to Structure > Blocks and choosing 'add new.' Once a block has been created, it can be set to be visible only for logged-in users (using 'Roles' link), and only on certain pages or only one page.
