You apparently once had a MobileMe account. When that closed you migrated to iCloud which replaced some of its facilities, principally syncing (not the iDisk, which terminated).
iCloud has a free storage level of 5GB for emails, calendars, contacts etc. As a concession to MobileMe subscribers you were given an additional storage level (normally paid for) and this terminates tomorrow.
If you have less than 5GB in iCloud and do nothing you will downgrade to 5GB but will notice nothing. If you have more than 5GB you can pay to continue your storage (Lion or Mountain Lion or iOS5/6 required).
If you wish to continue your current storage level, the instructions are here:
http://support.apple.com/kb/HT5527
Scroll down to 'How do I purchase my current plan?' and expand the section applicable to your computer or device.
If you would like to know more about iCloud please see this page:
http://rfwilmut.net/icloud