You probably already know that you can organize your songs into playlist folders. These playlists not only help you manage your catalog, but they also have corresponding playlist pages that you can pitch to and share with others. You can also add sub-folders to these playlist folders to add another level of organization. A good example would be an artist folder with the artist's albums as the sub-folders.
Create a sub-folder by right-clicking on an existing folder, and selecting New playlist folder from the drop-down menu.
Note: you can nest folders with sub-folders and so on up to 6 levels deep.