CMS
A content management system, often abbreviated as CMS, is software that helps users create, manage, and modify content on a website without the need for specialized technical knowledge.
HeadLess CMS
Headless Content Management Systems start the same. You can author, edit, and store content and data within the system, but the departure from a traditional CMS is in the templating. Instead of using the CMS templating system to create the page, all of that is offloaded onto a separate framework outside the CMS. This is accomplished through an Application Programming Interface (API), which allows content within the system to be accessible as data. That's a simplified version of the API concept, but all you really need to know is that rendering your web page is external to the CMS.
Strapi
Strapi is the leading open-source headless CMS. It’s 100% JavaScript and fully customizable.