Headless Drupal means using Drupal purely as a backend that exposes content through an API, while the front end is built with a separate framework like React, Vue, Next.js, or Gatsby.
This decoupling lets you use Drupal’s strong content modelling and editor experience while benefiting from modern front end tooling and performance. It is popular for mobile apps, progressive web apps, and sites where the front end needs to be independently deployable.
Drupal supports headless well thanks to JSON:API in core and mature GraphQL contrib support. The tradeoffs include losing some of Drupal’s built in theming benefits like automatic caching of rendered pages. You also need front end expertise on the team. For the right project, headless Drupal is a powerful architecture. For simpler sites, traditional Drupal remains easier and faster to build.