In technical terms, CMS is an abbreviation for Content Management System. Such systems represent website management tools which are widely-spread and readily available.
Managing a website used to require tech knowledge, web experience and other skills, but now complete beginners can build and manage big-scale websites without prior training, with nothing more than a simple hosting account.
Popular CMSs include WordPress, Magento, WooCommerce, Drupal, Wix, and Shopify.