An RSS Feed is a feed that users can subscribe to in order to be notified of updates as they happen. I use RSS Feeds and ThunderBird to subscribe to blogs and other news outlets so I don't have to open up my web browser and check on each one manually, it is especially disappointing to load a site only to find that there is no update for the day - multiply that experience by at least 5 web sites a day and 7 days a week. That's my not very technical definition.
Currently I am subbed to BlenderNation, Blender, GimpNews, and other class related blogs. It's very handy to check my email with ThunderBird and at the same time see that no new versions of my favorite open source programs have been released without visiting the official sites.
Here's a little article that is helpful for getting started: http://www.petefreitag.com/item/465.cfm
"Along with 'Antimatter,' and 'Dark Matter,' we've recently discovered the existence of 'Doesn't Matter,' which appears to have no effect on the universe whatsoever." -The 5th Wave