Where Drupal, PHP and SQL errors are logged. On a production server it is recommended that errors are only written to the error log. On a test server it can be helpful to write logs to the screen. By default, errors are displayed to the screen. You should change this to 'Write errors only to log' only for your production site.
You can tell Drupal to automatically discard error log entries older than the age you specify. This requires a crontab (example.com/cron.php) to be set up. See step 6 of the install.txt for example of setup.