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

<type name="Magento\Framework\App\FrontController">
    <plugin name="requestPreprocessor"
            type="Magento\Store\App\FrontController\Plugin\RequestPreprocessor"
            sortOrder="50"></plugin>
</type>

This is a type configuration.

This configuration is for the frontend area. This is the area that corresponds with the cart sub-application.

This configuration also tells the object manager to use the Magento\Store\App\FrontController\Plugin\RequestPreprocessor plugin class with Magento\Framework\App\FrontController objects.

<type name="Magento\Framework\App\FrontController">
    <plugin name="install"
            type="Magento\Framework\Module\Plugin\DbStatusValidator"
            sortOrder="40"></plugin>
    <plugin name="storeCookieValidate"
            type="Magento\Store\Model\Plugin\StoreCookie"
            sortOrder="10"></plugin>
</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 also tells the object manager to use the plugin class with Magento\Framework\App\FrontController objects.