Hi there!
The full name is something like The 49th Annual Gathering of the Rainbow Family of Living Light, if that helps capture it a little bit better. The ways that I describe it generally depend on the person, and how much they know about festival culture, anarchist theory, etc...
It's a huge, temporary, light-on-the-land (little to no petro/electricity around), fully decentralized, completely free (no $, no tickets, no company, no employees, no rules besides respect for sovereignty/consent - and basic rules everyone agrees to regarding conflict resolution & group decision-making) city that forms in the woods. The first one (1972) was has repeated ever since from July 1-7 (actually the ~8 weeks or more around that), and there are over 100 regionals, locals, internationals, and such each year, all operating through spontaneous order and conscious manifestation.
Another way of putting it is the title of my presentation/booklet: The Rainbow Gathering: Five Decades of Beta-Testing Anarchist Society
If you've got any particular questions, I'm happy to answer them.
Thanks for dropping in :-)