I once tried to explain RSS to my wife, who worked as a software engineer for years. After stumbling around trying to explain syndication and the XML format, I finally just signed her up for a Bloglines account and gave her my feeds. In other words, I couldn't explain it, even though it's a life-changing technology for me and I don't normally get tongue-tied over technology. Not long after that, I sat down with someone in marketing and tried to find a good explanation of RSS that we could "borrow" for our Web site. Not much luck there, either.
This blog post has the best explanation I've found, complete with this handy graphic.
There were a couple of snarky comments to the post, but I think most people appreciated it.
A shout-out should also go to Nintendowhiz for this post on the Nintendo NSider Forums about how to use RSS in their community.
I think RSS has the potential to change communities by removing the "walled garden" effect. One feature I really like in our app is that you can take an RSS feed of all of a user's posts. So if I post a lot of messages on a community, I can take an RSS feed of those messages and display it on my personal home page, so such profound thoughts as I have can be shared not only with the community, but with anyone who visits my site.