GraphCMS breaks API's into two categories: a management API and a content API. As the name implies, management API's pertain to everthing that has to do with the management of your projects (all your projects) and their configuration. The content API relates to all of the content scoped to an individual project.
A walk through of the concepts you might want to understand when working with GraphCMS APIs.
Documentation of the Content API includes how to query, mutate and more. This is the main API.
Documentation of the Management API includes functionality surrounding your account, projects and more.