CMS stands for Content Management System. It is an umbrella term for any software that is used to manage the creation and modification of digital content, such as websites and all their associated content.
With a CMS, you can publish, organise, change, or remove various kinds of content, like text, images, video, interactive code, etc, without modifying code. The CMS provides a user interface for you to build and manage your website content more easily, as opposed to editing the website code directly.
Self-hosted CMS
These are CMS' that you can download and host on your own server with the most popular example, WordPress.
Other examples include Joomla, Drupal, Magento, etc.
Website Builders
These are CMS services that you can subscribe to, hosted and managed by their respective parties.
Examples include Shopify, Squarespace, Wix, etc.