CMS is a content management system. To be more simple, a content management system is a tool that helps you in building a website without needing to code from scratch.
It is an alternative to developing your own system to create web pages, store images, and other functions, the content management system manages all that basic infrastructure stuff for you, allowing you to focus on more forward-facing parts of your website.
Apart from websites, you can also use content management systems for other functions such as document management.
There are several CMSs available online but the most popular one is WordPress since it's an open-source platform.