This means that the Group ownership field has a Group ID that is not contained in /etc/group, so the Finder just substitutes 'unknown'.
There are 4 billion possible Group ID numbers, but only a handful are ever used. Somehow you have stored a file on your system that is using a Group ID that does not contain a name in /etc/group
For example, if you received a ZIP file (or any other container file) and unpacked it, it is possible that the Group ID from another system was used when creating the file on your Mac.
You can use Applicaitons -> Utilities -> Terminal to change the group owner
sudo chgrp staff "/path/to/the/file.name"
Or you can just ignore this, as it is most likely harmless
Message was edited by: BobHarris
Hmm, ok. So if I ignored it, it doesn't mean someone will be able to access my computer in the future—like some clever trojan or something? This "unknown" sometimes has the ability to read and read/write (iirc). I am unable to change it to No Access unfortunately.
I don't understand the terminal line. What would the terminal line change the group owner to, and what is "file.name"?