It’s a command line shell and Unix interface for Drupal developers to make it easier to work on outside of browsers. The commands are interacting with code like modules/themes/profile and other command such as
drush sqlc cron and clean cache jobs etc.
composer if you haven’t already
curl -sS https://getcomposer.org/installer | php
$mv composer.phar /usr/local/bin/composer
ln -s /usr/local/bin/composer /usr/bin/composer
drush from git repo
git clone https://github.com/drush-ops/drush.git /usr/local/src/drush
Get the latest one
cd /usr/local/src/drush git checkout master #for the latest one or whatever version you want
ln -s /usr/local/src/drush/drush /usr/bin/drush
composer install #if this results in error, run composer update to add the dependencies
Check the version
Have fun with
drush within the folder where you installed Drupal.