<preference for="Magento\Framework\App\FrontControllerInterface"
            type="Magento\GraphQl\Controller\GraphQl"></preference>

This is a preference configuration.

This configuration is for the graphql area. This is the area that corresponds with the GraphQL sub-application.

This preference configuration tells the Object Manager when a user wants a Magento\Framework\App\FrontControllerInterface object that it should return a Magento\GraphQl\Controller\GraphQl object.