A Content Management System (CMS) does what it says on the tin. A CMS system, such as WordPress or Joomla, is a software application on which you can build and manage a website.
One major selling point for a CMS is that you do not need to know how to code in order to build your website. While the CMS application takes care of code and infrastructure, you are able to focus on optimising the front-end of the website.