"A collection is a group of subscriptions you can create to help you organize and view content from the channels you're subscribed to. Collections can be created by themes (like 'basketball' or 'music')," explains YouTube.
Collections could be created, deleted and edited from the subscription manager. In many ways, YouTube collections were just like folders in a feed reader.
If you want to use a feed reader to manage your YouTube subscriptions, you can export them to OPML and import the file into your favorite feed reader. Open the subscriptions manager, scroll down to the bottom of the page and click "Export subscriptions". Another options is to use this link.