WordPress is a CMS.
Content management systems pretty much do as the name suggests, they allow you to build and manage the content of your site and publish it to the internet.
WordPress is the most popular CMS on the planet, with over 30% of sites on the internet (at the time of writing) using WordPress as their CMS.
WordPress is also open source, so it makes a great platform for WordPress agencies to develop on.
CMS can also help non-developers to create a simple site without adding any code, making it really useful for a lot of people and businesses.