RSS is very difficult to explain to someone who hasn’t seen it in action. I end up describing it (very generally) like so:
RSS is a way that a web site can make information (articles, train schedules, music, etc.) available in a technically standard way. There is a wide variety of a type of application called “reader” or “aggregator” out there that will go out, find the RSS information you tell it to and then display it all on in one easy place for you to read (or listen to). These readers are available as an application you install on your computer, just like installing something like WinAmp or Google’s Picassa, or you can use an online reader like Bloglines with no installing necessary.
I am totally addicted to my Bloglines account because I can’t keep up with everything I want to read without something helping me out with the hunting, gathering and cooking so I can consume with relative ease (hm, that may explain the information weight-gain).
Of Zen and Computing is a site that has a pretty basic explanation of RSS as well as being a site with lots of approachable explanations for many aspects of computers.