Tail is command for Unix and Unix-like systems (like OS X) that allows you to take a peek at the contents of the end of a file. From the manual page: "tail - output the last part of files." Tail can be really useful for debugging purposes, or for taking a look at the recent access_logs from your Drupal setup. Tail can be particularly useful in a production environment when you may not have PHP error reporting enabled, and need to find the cause of serious errors with your Drupal site.