CMS stands for Content Management System

Traditional CMS

A traditional CMS is a Content Management System that is one big monolith. a single server handles rendering, the presentation, fetching data from DB, etc...

Headless CMS

it is a way to describe an app that has it's backend separated from it front-end and they talk between each other with a REST API.
The CMS is only responsible for managing content and not the presentation
Better performance than a regular CMS. faster lading.
framework agnostic.
scalable. front and back-end are decoupled.
future proof bc it separates the content from the presentation.