<type name="Magento\Elasticsearch\Model\Adapter\Index\Config\EsConfig">
    <arguments>
        <argument name="reader" xsi:type="object">
            Magento\Elasticsearch\Model\Adapter\Index\Config\Reader
        </argument>
        <argument name="cacheId" xsi:type="string">
            elasticsearch_index_config
        </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\Elasticsearch\Model\Adapter\Index\Config\EsConfig object that the following arguments should be used in the foo object's constructor.

• $reader = new Magento\Elasticsearch\Model\Adapter\Index\Config\Reader (...)
• $cacheId = "elasticsearch_index_config";

<preference for="Magento\Elasticsearch\Model\Adapter\Index\Config\EsConfigInterface"
            type="Magento\Elasticsearch\Model\Adapter\Index\Config\EsConfig"></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\Elasticsearch\Model\Adapter\Index\Config\EsConfigInterface object that it should return a Magento\Elasticsearch\Model\Adapter\Index\Config\EsConfig object.