What is the «GraphQl» (`Magento_GraphQl`) module for?

GraphQl provides the framework for the application to expose GraphQL compliant web services.
It exposes an area for GraphQL services and resolves request data based on the generated schema. It also maps this response to a JSON object for the client to read.

github.com/magento/magento2/blob/5e00970c/app/code/Magento/GraphQl/README.md

See also:

• Since 2017-10-25, Magento 2.3 includes a new «GraphQl» (Magento_GraphQl) module

• Since 2017-12-04, Magento 2.3 includes a new «GraphQlCatalog» (Magento_GraphQlCatalog) module.
• Since 2017-12-04, Magento 2.3 includes a new «GraphQlConfigurableProduct» (Magento_GraphQlConfigurableProduct) module.
• Since 2017-12-04, Magento 2.3 includes a new «GraphQlEav» (Magento_GraphQlEav) module.
• Since 2017-12-08, Magento 2.3 includes a new «GraphQlCustomer» (Magento_GraphQlCustomer) module.