Drupal is typically called a CMS (content management system) - somewhere you add and edit the content for your website.
However, unlike some other CMSes, unless a Drupal website uses only the default page and article content type, no Drupal websites are the same. Each will have its specific content model with different content types and fields.
Instead of being a content management system, Drupal can be described as a content management framework (CMF) - a framework for building a content management system specific to each application.
But as well as a content management framework, Drupal could also be considered an application framework like Symfony or Laravel.
As well as building the content model, you can extend Drupal’s functionality with custom modules and add custom themes to make it visually different.
But, there’s a lot of functionality in Drupal core itself - such as user accounts, authentication, a query builder (Views), JSON:API endpoints, and a visual page editor (Layout Builder) - to name a few things!
Drupal offers much more than just managing content.