<type name="Magento\Framework\Logger\Monolog">
    <arguments>
        <argument name="name" xsi:type="string">
            main
        </argument>
        <argument name="handlers" xsi:type="array">
            <item name="system" xsi:type="object">
                Magento\Framework\Logger\Handler\System
            </item>
            <item name="debug" xsi:type="object">
                Magento\Framework\Logger\Handler\Debug
            </item>
        </argument>
    </arguments>
</type>

This is a type configuration.

This configuration is the application level configuration (`app/etc/di.xml`) fo the object manager. It is not a part of a specific module, but will apply to all sub-applications in Magento 2 (cart, admin, api, cron, etc).

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

• $name = "main";
• $handlers = [...items...];

<preference for="Psr\Log\LoggerInterface"
            type="Magento\Framework\Logger\Monolog"></preference>

This is a preference configuration.

This configuration is the application level configuration (`app/etc/di.xml`) fo the object manager. It is not a part of a specific module, but will apply to all sub-applications in Magento 2 (cart, admin, api, cron, etc).

This preference configuration tells the Object Manager when a user wants a Psr\Log\LoggerInterface object that it should return a Magento\Framework\Logger\Monolog object.