<preference for="Magento\Framework\GraphQl\ConfigInterface"
            type="Magento\Framework\GraphQl\Config"></preference>

This is a preference configuration.

This configuration is for the base area. Base area configurations will apply to all the sub-applications.

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