Represted with ( ) icon, folders--also called bins or in general containers--are the building blocks of the folder hierarchy where all your subscriptions are kept. They are used to categories your subscriptions as you do with your folders and files on your computer. You can manage folders from the 'Subscriptions' tab, that is where existing folder hierarchy is shown.
Apart from building hierarchy, you can also use folders like containers to keep posts. You can copy and move posts from other feeds into the folders for archive purposes.
To learn more about folders, check out related help topics.