RSS stands for Really Simple Syndication. It's a way for you to "subscribe" to frequently-updated web sites such as blogs, and be notified any time there is new content available on each site. Further, you can use an "RSS Reader" application to manage all of your subscriptions, and to read all of the new content from within the reader application.
Why bother with using an RSS reader instead of just directly visiting the website of each individual blog that you like to read? Here are two pretty good reasons:
(1) You don't have to spend time visiting a site only to find that it hasn't been updated since your last visit.
(2) It's faster to skim/read all of the new content from the sites that have been updated from a single centralized location, instead of having to visit them all individually.
How do you know if a site provides an RSS feed of its content? Such sites will generally display a small orange RSS icon somewhere on the page: In Firefox, the RSS icon be displayed at the right end of the browser's address bar when you're at a site that provides an RSS feed. You can generally copy-and-paste the site URL into your RSS reader to subscribe to the site's RSS feed. In Firefox 2 you can just click on the orange icon in the address bar, and it will automatically handle the subscription for you, if you already have an RSS reader program set up.
Here are a couple of links for you to get started with. The RSS reader I use is a free web-based application called Bloglines, located at http://www.bloglines.com One of my favorite blogs to read is Jeff Atwood's "Coding Horror" technology issues blog at http://www.codinghorror.com/blog/.