They are not to entertain anyone except the participants. Make it fun. If you have to, work hard to make it fun. If it isn't fun, don't do it. Rule 2 .
Jams are the time to make mistakes.
In a Jam you can experiment, screw up, fall down, drop your axe and otherwise make a fool of yourself. The flip side of this is that you have to be tolerant of others who are screwing up. In any case good manners should be as important here as in any social situation. Rule 3 . Jams are for beginners as well as experts.