Autoconfirmed modchat is having an username for at least a week and having won at at least one match.
It is implemented, I believe, to help stop spammers, although it's not foolproof, as IP changes can use the same username as before to spam and then change IPs, circumventing the point of aurtoconfirmed modchat as the usernames fill the requirements.
It cannot be left on at all times, due to the fact that newer members that either haven't played for a week or cannot win are unable to ask for help.