It means that neither of the participants have tied their self-worth or identity into what is being argued. It means that evidence and logic will be the only criteria for argument-- not emotions, not personal, private preferences, not warm fuzzies. It means that both parties will avoid logical fallacies and will admit it and change when the other party points one out in their argument. It means objectivity-- the willingness to change one's own mind in the face of superior evidence and not squirrel away their beliefs and protect them from scrutiny.
Basically, it's a discussion in which logic is the most important-- not winning, not ego, and not anything personal.