As you can see, three sets (Home, Work, Vacation) have been created by using the Manage Bookmark-Sets action from the bookmarks view. Each set contains a list of bookmarks. You can switch between sets using the two yellow arrows in the toolbar of the view. Navigating between sets will make the list of subscriptions reduce to only those that are part of the selected set. You can move bookmarks to other sets either by opening the Properties of the bookmark and changing its location, or dragging it into another set inside the opened Manage Bookmark-Sets dialog.
Sets can also be useful if you have some bookmarks that you want to keep, but remove from your active set of subscriptions. Simply move them to another set. Once you changed your mind, you can move those bookmarks back to your active set.