Drush stands for Drupal shell, which provide shell interface for managing Drupal sites. It is a useful command line tool to perform various admin tasks using just one or two commands in the terminal. It seems very complex for newbie but when you will get familiar with Drush then it provides easy handling of Drupal sites including installation, Updating and for performing lots of admin tasks like enable/disable modules, clearing cache etc.
For using Drush, you only needs to install Drush and after that you can operate your Drupal site.
Use following command to install drush in you Ubuntu OS: