<preference for="Magento\Framework\App\ProductMetadataInterface"
            type="Magento\Framework\App\ProductMetadata"></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 Magento\Framework\App\ProductMetadataInterface object that it should return a Magento\Framework\App\ProductMetadata object.