Basically, it allows you to "subscribe" to a websites CONTENT and get updates sent to you automatically, saving you the efforts and memory of going to a site and checking for something new.
To subscribe, you click on the RSS symbol (this can look differently on different sites)
and that takes you to a line of code that will look something like this "
I've borrowed this info from Beachbody.com (I'm a happy client, but also, I liked their explanation)
What is RSS?
Large RSS IconRSS stands for Really Simple Syndication. Using RSS there is no need to constantly check for updates across multiple Web sites. As soon as articles are published they are gathered into one easy-to-read list.
How do I use RSS?
To subscribe to an RSS feed you should get a feed reader or "news aggregator," which will check for updates. For Windows you can use FeedDemon, FeedReader, or SharpReader. For Mac OS, there is Newsfire or Safari's built-in support. There are also integrated readers for Internet Explorer (Lektora) and Firefox (Sage). Bloglines is an online news aggregator. There are many more RSS readers. After you have installed your reader, follow the instructions for how to subscribe. Most of the time you can just click on the Web site's RSS link once the software is running.