Drush stands for Drupal Shell. It is a command line tool that lets you manage Drupal sites from the terminal instead of clicking through the admin interface.
With Drush you can install and enable modules, clear caches, run database updates, export and import configuration, and manage users. It dramatically speeds up common tasks and is essential for deployment workflows.
For example, running cache rebuilds and config imports through Drush takes seconds compared to minutes in the UI. It is installed per project via Composer rather than globally, which keeps it aligned with the Drupal version you are running. Any developer working on Drupal seriously should learn Drush early. It quickly becomes second nature.