A group of people who gather together regularly - usually once a year - to live in harmony with each other and nature. Money and alcohol are not allowed, but sharing and swapping are encouraged. It's basically a chance to step out of the commercial, high-speed world (or "Babylon" as it's been termed) and live simply and sweetly out in the woods with people who want peace and prioritise ecological issues.
The biggest Rainbow Gathering is in America, held annually in one of the national forests. The website is www.welcomehome.org and you can find lots of info about hows, whens, wheres and whys.
And to reiterate: welcome home!
When the earth is ravaged and the animals are dying, a new tribe of people shall come unto the earth from many colors, classes, creeds, and who by their actions and deeds shall make the earth green again. They will be known as the warriors of the Rainbow.
- Old Native American Prophecy
And there you have the point of a rainbow gathering!