Drupal is a content management system (CMS) which can be compared to WordPress. A CMS is a way of creating and keeping track of different webpages and online content in a dynamic way. It is said that if you can't do what you want with WordPress, you can do it in Drupal. While there is a 'Drupal core', which will give you a functioning website, there are also many modules that may need to be added and configured to add more features. If you are new to creating websites this may not be for you, but if you have a clear idea of what you need then Drupal allows you to create a website or service relatively quickly.
Creating websites with complex requirements, including many different kinds of content
Setting up membership websites with different levels of access
A very rich and flexible framework for creating websites
Easy to get lost in the complexities if you are new to creating websites