Quick explanation: It's a protocol for delivering "news feeds". It is currently used to notify or summarize new news stories, new blog articles or in this case new forum messages to another websites or a user's RSS reader for syndication to other websites or aggregation for a user.
The RSS links I've listed will provide the 10 most recent posts or the 5 most recent topics/threads, but even though the URL looks like a web address the format is XML/RSS so only an RSS-aware program can do anything useful with them.
I think most news, magazine and blog sites have RSS feeds available now. Look for small print about RSS, syndication or "news feeds". Several non-Microsoft web browsers have RSS reader support, and there are standalone programs. Web portals like Yahoo! can be configured to aggregate stories for you via RSS similar to the way the link to my site above does.
The protocol is more of a data format than a layout format meaning there is no common user interface, so the way you use it varies more widely than say WWW surfing.
The RSS data feed from Early Retirement Forums is a feature of this forum software we switched to in May this year.
Some additional babbling: Do you recall the program Pointcast or the Active Desktop Channels in Windows95? RSS is somewhat similar but is a "pull" content delivery mechanism rather than "push" like the predecessors I mention. This means instead of having info shoved at you after installing Pointcast or Active Desktop Channels RSS only gives you info when you request it. AvantGo is another example, although I'm not sure if you'd call that push or pull delivery.
One last babble: The RSS-provided content is not necessarily limited in size but is frequently limited in size, so you may only see the first part of a post/article/etc., but there is always a link directly to the article on the publishing site for further reading. The length limit is configurable on this board software, but I don't recall what it's set to.