A headless CMS means it only provides data and methods through an API instead of being part of a template engine and SDK. By separating data from the application language it can be used as a data source for any language or technology.
Strapi can be configured to work with (almost) any database of your choice. And can be set up using either MySQL, PostgreSQL, MongoDB or SQLite.
With its plug-in system and configurable setup and admin panel, Strapi is highly configurable to suit your needs. All data types are created from scratch with a set of predefined formats and data relations.
For an author, using the CMS is really easy since it supports many data types which can be combined to create any data structure. It also allows grouping repeatable fields in components and support relationships between types.
Out of the box, Strapi provides a full REST API for all your collections with GET, POST, PUT and DELETE endpoints. The permission editor defines what actions are available for each user on each collection.
It also fully supports GraphQL via a plug-in which automatically generates a schema based on collections, types and methods.