Is this still a problem for you? The root folder is where you are going to store your media files that Sonarr downloads. The first thing you need to do is create the folder in your filesystem such as /home/tv_shows or some other place where you would like to keep them.
Then go to Settings / Media Management
In here you can configure the /home/tv_shows folder as a root folder. What Sonarr will do is whenever you add a new series it will ask you for the root folder for that series (in my case I have two different root folders for TV) and will create the series folder in your file system as a subdirectory to the root you selected. If you are moving a Sonarr configuration between two different servers you need to make sure that the root folder exists on the new server or you will have to edit the root folder for all series that were part of the previous configuration to point to your new root folder.