Drush is a command line shell and Unix scripting interface for Drupal. If you
are unfamiliar with shell scripting, reviewing the documentation for your shell
(e.g. man bash) or reading an online tutorial (e.g. search for "bash tutorial")
will help you get the most out of Drush.

Drush core ships with lots of useful commands for interacting with code like
modules/themes/profiles. Similarly, it runs update.php, executes sql queries
and DB migrations, and misc utilities like run cron or clear cache.