This would contain the character set being used by the document. For example the value "iso-8859-1" is likely to be returned, but the local variant of the browser and OS may affect the value you get.
This property might contain a value such as:
Details of other aliases can be located at the IANA registry. In that registry are listed the names and aliases of a wide variety of character sets. Even though there are nearly 800 names and aliases, it seems on inspection that there are items missing.