The terms account and sub-account are organizational units within Curve. Every instance of Curve has the potential to contain a hierarchy of accounts and sub-accounts but starts out with just one account (referred to as the top-level account).
Once a hierarchy has been created, admins can be assigned to specific sub-accounts to limit their scope of what they can change as well as making the courses and people they manage more specific and relevant to them.
Permissions trickle down through the hierarchy but not up. Admins at one level (or account) in the hierarchy have admin permissions at that level as well as in any sub-account of that account. Admins can move courses within their sub-accounts, but not between them unless they are also the admin of the parent account of each sub-account.
With the exception of Terms, SIS Imports, Authentication and a few account settings, most settings and items can be modified or, in the case of permissions, overridden in a sub-account.