GraphQL is a data query language developed internally by Facebook in 2012 before being publicly released in 2015. Magento implements GraphQL to provide an alternative to REST and SOAP web APIs for frontend development.
How to access GraphQL
Use a GraphQL IDE such as GraphiQL or a browser extension to run the code samples and tutorials. If you install a browser extension, make sure it has the ability to set request headers. On Google Chrome, Altair GraphQL Client is one extension that can do the job.
To begin exploring GraphQL, set the GraphQL endpoint by entering
http://<magento2-server>/graphql in the URL bar of your IDE or extension. You can use the browser in the right column to determine how to set up a query or mutation. Examples are also available throughout the Magento GraphQL documentation.
The following image shows a sample query, its response, and the GraphQL browser: