A Node.js headless CMS is a content management system that doesn't have a front-end user interface. It will typically only have a back-end user interface where an admin will login in manage content and site assets such as images and videos.
There is a wide range of difference between the many headless CMS out there. Some simply allow you to manage JSON documents, while other offer advanced management of content types and entail advanced media management tools as well.
So, since there is no front UI/UX, how is the content used? The content and media is exposed exclusively through an API. The most modern API protocols for headless CMS are REST and Graph QL.
Both REST and Graph QL based APIs are good options assuming your requirements are somewhere between simple and moderate in complexity. REST has been prevalent and the clear standard in API protocol for well over a decade.
Graph QL was introduced in 2015 and is just starting to gain traction. However, just because its newer, doesn't mean its a better fit for your project. If you are starting a greenfield project, it is certainly prudent to take a look a both options and determine which is more future-proof for your specific requirements.
SonicJs can easily be used as a node based headless CMS. Because SonicJs is 100% API driven, all the content types that you create from the admin user interface are automagically accessible via the built-in API end points.
All common CRUD operations plus common utilities are supported without any coding. This makes SonicJs a great choice for managing content for you mobile, web, or IoT project.