CMS is an acronym meaning “Content Management System”.
In other words, a CMS is a software that facilitates the creation, editing, organization and publication of content.
WordPress is therefore a CMS since it allows you to create and publish your content on the web. Although it is most often used to publish on the internet, it can also be used to manage content on an intranet, or even locally on a computer.
WordPress allows users to have complete control over the database, files, documents, and the design and display of content. No coding skills are required to use it.
Once you install a CMS, you will own your site. This also implies that you will be responsible for subscribing to a hosting provider, setting up the site and also managing its security.
Today, WordPress is the most popular CMS, with more than 40% of sites worldwide using it (source).
