Seth's all about spreading ideas and has written at length about getting remarkable ideas to spread. The concept of tribes expands on this -- it's about creating a platform so it's easy for people (your tribe) to spread your ideas.
I've spent quite a bit of time this summer working on our local community association website and have been trying to embrace the tribe idea. But I'll leave that for another post. For now, download Seth's ebook , give it 3 or 4 good reads and tell me what you think in the comments!