What is the «GraphQlCatalog» (`Magento_GraphQlCatalog`) module for?

GraphQlCatalog provides type and resolver information for the GraphQl module to generate catalog and product information endpoints.

github.com/magento/magento2/blob/bce891d2/app/code/Magento/GraphQlCatalog/README.md

See also:

• Since 2017-12-04, Magento 2.3 includes a new «GraphQlCatalog» (Magento_GraphQlCatalog) module.
• Since 2017-10-25, Magento 2.3 includes a new «GraphQl» (Magento_GraphQl) module.