Mapping parties are community events targeted at getting a specific area mapped in a day or a weekend. You come along, are loaned a GPS unit and shown how to use it (it's easy!) and you go out mapping an area of your choosing. Then you come back and are shown how to get that data in to the map.
They are relaxed events and you're supposed to have fun. Often we grab a beer after the event, and this is no exception.
Official Website: http://wiki.openstreetmap.org/index.php/Dallas