A CMS is a Content Management System. If want to make any changes or updates to the content on your website, you’ll do this in the CMS. In other words, it’s the back end to your front end.
There are free and paid-for CMSs, and it's best to select one that best suits your needs and your technical ability. e.g. Do you need your website to be transactional? Can you code HTML or do you need your CMS to be WYSIWYG (What You See Is What You Get)?
Popular CMSs are Wordpress, Drupal, Joomla!, Ghost, Squarespace, Magento, Wix, Adobe Experience Manager.