<type name="Magento\Framework\GraphQl\Config">
    <arguments>
        <argument name="data" xsi:type="object">
            Magento\Framework\GraphQl\Config\Data
        </argument>
    </arguments>
</type>

This is a type configuration.

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

This configuration tells the Object Manager that when a user wants a Magento\Framework\GraphQl\Config object that the following arguments should be used in the foo object's constructor.

• $data = new Magento\Framework\GraphQl\Config\Data (...)

<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.