syndie.de explains itself pretty well if you click on the links in its sidebar -- lots of self-explanatory material.
It's one of several efforts to fight oppressive intrusion & snooping by developing peer-to-peer social networks that will be much more difficult to shut down than grabbing a central server with a search warrant or confiscating a domain registration. With our FBI wanting back doors into everything, NSA sniffing everything that passes by, and DHS doing it best to emulate its East German namesake, the Stasi, these efforts become all the more important.
Another major effort that I'm aware of is AnonPlus.
Now if you could build a product as slick as XenForo on top of one of these anonymous networks ....