Archived data is any data that has been deleted from your local drive but is still stored in your Vault Services account.
For example, let's say you schedule a daily automated backup job for your "My Documents" folder.
The backup runs according to schedule on 3/14 at 3:00 pm.
On 3/15 at 8:00 am, you delete 5 files from your "My Documents" folder on your local machine. If your settings are such that your backup job retains any files that have been deleted locally from your computer, your Vault Services account will continue to store these 5 files and they are classified as "Archived" data.