It means that you shouldn't be relying on Apple-provided file storage services for your important documents. They have gone through several iterations of services like iDisk, MobileMe, etc. and iCloud could well suffer the same fate one day. Use Dropbox or another service that's designed to integrate well with your Mac or PC so you can easily maintain your own backups and easily copy your files to any other service you may choose in the future.
iCloud is particularly bad at this because provides access to files only on an app-specific basis, rather than a generic remote filesystem that you can access directly from your computer, which is what most other services (like Dropbox or support in providing WebDAV e.g. box.com) provide.