Walks through the process of enabling the theme developer module and showing the themer info, which is like Firebug for Drupal theming. You can choose specific elements on the page to see what Drupal template files or theme functions were involved with outputting it to the screen. You can easily look at the candidate template names that are available, as well as see all of the variables that are available from that template file. A list of documented variables can be found within the node module's node.tpl.php file, but sometimes there are undocumented variables that are coming from contributed modules or elsewhere. It's a super handy tool to have available, but only enable it when you really need it since it can adversely affect the mark-up on your page and cause some wonky behavior.
The Theme Developer module is on Drupal.org at http://drupal.org/project/devel_themer. Once you find a module on Drupal.org, you can see the "machine name" for the module in the URL (it is the same name as is used to for the module's .info file as well). That is the name that Drush uses. So, in this instance, Drush is looking for "devel_themer".
drush dl devel_themer