These charts give information about your membership statistics over time—for example, subscribes, unsubscribes, confirmed, or on hold.
These charts can give you general trends about your list's membership. For example, you may find that your rate of unsubscribes peaks after a particular kind of mailing, and so may decide that it's not worth doing that kind of mailing if it means you will be sacrificing so many members.
You may also find studying confirmation rates over time useful, so you may tailor your confirmation document to encourage more people to confirm joining your list.
Email addresses go bad and are put on hold as a matter of course in running an email list. However, if you find that a large number of email addresses go on hold all at once in these charts, you may investigate ISP or network problems.