Piwik Autologin is a Drupal module which logs you instantly into Piwik when you log into Drupal.
For this you have to give to Drupal the Piwik credentials (login and MD5's password) of a valid Piwik user on the Piwik install which tracks your site. Note that the corresponding user should be granted the 'view' or 'admin' rights on this website on Piwik. It sholud not be a Piwik superuser too.