A CMS (Content Management System) lets you add and edit site content (text, images, pages) via a browser-based panel — without writing code. An editor logs in, selects a page or post and changes the content.
WordPress is very popular: easy to use, many themes and plugins. It suits blogs, company sites and smaller shops (with WooCommerce). Drupal is more advanced and flexible, often chosen for complex sites and large organisations.
The right CMS depends on your goals, site size, requirements (e.g. multilingual, integrations) and who will edit content. WordPress is often enough; for specific needs, Drupal or another solution may be better.