A charter can be legally binding, such as in the magna carta etc. and it can also refer to a pledge to meet certain standards of service which may or may not be legally binding, for example, train companies in the UK have a passenger charter which is a pledge to run trains on time and to certain minimum standards of punctuality, comfort and cleanliness etc.

Thanks so much! The article was in the Guardian spoked about press's rules

I'm not sure what context it's being used in, but.. Charter-backed = Having the legal authority from an institution (university, city etc) to do 'something' In the case of an organization, a charter defines it's rules and regulations. Therefore, being charter-backed is simply the equivalent of being set rules by a higher power and then being given permission to follow those rules.