It stands for 'Really Simple Syndication'. It lets sites send out recent updates on new articles and happenings. Sometimes, they can offer different sections of the site as different RSS feeds. So, you can choose the areas you want to keep up-to-date on by subscribing to one or more particular RSS.
You can click on this next to a webpage address in your browser and subscribe to get these lists of updates (usually the titles of articles, or short summary).
In Firefox, for example, you can add a page's RSS feed as a bookmark folder - then every 10 or so minutes it updates automatically with the latest list of items from the page your subscribed to. I use this on MR to save me needing to go to the main page. I just click the bookmark folder, read what the latest updates are from ppl and I can click any I want to read and jump straight to the page in MR with it on.
You can also get RSS feeds from different sites to show as one page with short summaries of each update/article saving you having to go to many different sites and checking what's happened recently on each separately. I don't do that myself but you can do it from IE 7. Just type 'RSS feed tutorial' in Google and you'll get step-by-step on how to do that too. =)