<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.

<preference for="Magento\Framework\App\FrontControllerInterface"
            type="Magento\Webapi\Controller\Soap"></preference>

This is a preference configuration.

This configuration is for the webapi_soap area. This is the area that corresponds with the SOAP Api sub-application.

This preference configuration tells the Object Manager when a user wants a Magento\Framework\App\FrontControllerInterface object that it should return a Magento\Webapi\Controller\Soap object.

<preference for="Magento\Framework\App\FrontControllerInterface"
            type="Magento\Webapi\Controller\Rest"></preference>

This is a preference configuration.

This configuration is for the webapi_rest area. This is the area that corresponds with the REST Api sub-application.

This preference configuration tells the Object Manager when a user wants a Magento\Framework\App\FrontControllerInterface object that it should return a Magento\Webapi\Controller\Rest object.

<type name="Magento\Framework\App\FrontControllerInterface">
    <plugin name="front-controller-builtin-cache"
            type="Magento\PageCache\Model\App\FrontController\BuiltinPlugin"></plugin>
    <plugin name="front-controller-varnish-cache"
            type="Magento\PageCache\Model\App\FrontController\VarnishPlugin"></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 plugin class with Magento\Framework\App\FrontControllerInterface objects.