Drush is a command line shell and scripting interface for Drupal. It can only be used on shell hosting accounts for this reason. You can read more at the Drush home page.
Drush is very simple to install and get going. Visit the
Drush home page
and follow the "Download" links to find your preferred version. Copy the link
for the latest reccomended release's tar.gz file to your clipboard and download
the file to the root of your drupal site using the command
link]. You should have just downloaded a file with a name similar to
drush-7.x-5.7.tar.gz. You then need to extract the file with the
tar xzvf [drush file].tar.gz. Use the
command to check that a drush directory has been created. You can now remove
the .tar.gz file using the command
rm [drush file].tar.gz.
Change into the Drush directory using the command
cd drush. You
should be able to execute Drush using the command
instructions on using Drush, see the documentation linked from the
Drush home page.