It's essentially a back-end only content management system that makes content accessible via a RESTful API.
According to wikipedia, the term "headless" comes from the concept of chopping the "head" (the front-end, i.e. the website) off the "body" (the back-end, i.e. the content repository).
According to wikipedia, the term "headless" comes from the concept of chopping the "head" (the front-end, i.e. the website) off the "body" (the back-end, i.e. the content repository).