Composer is the standard dependency manager for PHP. Drupal uses it to manage core, contributed modules, themes, and third party libraries in a single unified workflow.
When you add a module, Composer pulls in the code along with any required dependencies and updates a lock file so every environment stays in sync.
This approach replaced the old habit of manually downloading modules and dropping them into folders. It brings Drupal in line with modern PHP development and makes upgrades, security patches, and team collaboration much cleaner. A typical Drupal project today lives in a Git repository containing a composer.json file, with the actual code fetched during deployment. Learning Composer basics is now essential for any Drupal developer.