<preference for="Magento\Ui\Config\Argument\ParserInterface"
            type="Magento\Ui\Config\Argument\Parser\Composite"></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\Ui\Config\Argument\ParserInterface object that it should return a Magento\Ui\Config\Argument\Parser\Composite object.