<type name="Magento\Store\Model\Config\Reader\ReaderPool">
    <arguments>
        <argument name="readers" xsi:type="array">
            <item name="default" xsi:type="object">
                Magento\Store\Model\Config\Reader\DefaultReader
            </item>
            <item name="website" xsi:type="object">
                Magento\Store\Model\Config\Reader\Website
            </item>
            <item name="websites" xsi:type="object">
                Magento\Store\Model\Config\Reader\Website
            </item>
            <item name="store" xsi:type="object">
                Magento\Store\Model\Config\Reader\Store
            </item>
            <item name="stores" xsi:type="object">
                Magento\Store\Model\Config\Reader\Store
            </item>
        </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\Store\Model\Config\Reader\ReaderPool object that the following arguments should be used in the foo object's constructor.

• $readers = [...items...];

<preference for="Magento\Framework\App\Config\Scope\ReaderPoolInterface"
            type="Magento\Store\Model\Config\Reader\ReaderPool"></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\App\Config\Scope\ReaderPoolInterface object that it should return a Magento\Store\Model\Config\Reader\ReaderPool object.