A folder view is a way how Windows Explorer show the content of a folder. Open a folder, right click in the blank area, you will see a "View" menu item with several sub menu items, such as "Thumbnails", "Tiles", "Icons", "List" and "Details". For more information see Microsoft Knowledge Base KB812003.
To change the view of a folder using FolderViewSet, you have to close all windows of the folder first. It's because Windows Explorer will remember the folder's view when it's last opened.
This is a bug of Microsoft. You can find a solution at Microsoft Knowledge Base KB813711.