A Twunch is an informal Twitter lunch. The location is a restaurant where one can eat fast and cheap without bookings. If you want to propose a Twunch tweet a location and time and include @twunch. If you want to join a Twunch just reply @twunch so that everyone knows who will be there.
The Twunch iPhone app does exactly that, it scans for future Twunches and shows the one nearest to your current location. You can than see who will be there, where it will be and when exactly you'll need to show up. You can subscribe from within the app and find the Twunch' location on a map.