That string is a Security Identifier, aka SID. Every security principal in Active Directory (users, computers, groups, etc.) has one, Under normal circumstances, Windows will contact a DC to query the name for the SID when you view permission ACLs, and you would see the username, e.g. YOURDOMAIN\username.
Typical causes for a SID being displayed instead of a user/group/computer name are (in decreasing order of likelihood):
- The SID belonged to a security principal which has been deleted.
- A DC cannot be contacted for some reason. In your SBS environment this is highly unlikely to be the cause. Also, if this were the cause, you would see all ACEs use a SID rather than a friendly name, not just some.
- The SID belongs to a security principal (user, etc.) in a remote trusted forest and there are connectivity issues. I don't have a lot of SBS experience so this may not even be possible.