"RSS" can stand for several different things, including Really Simple Syndication, Rich Site Summary, and RDF Site Summary.
RSS is basically a 'feed" of information from a website. Rather than visiting a website to look for new content, an RSS feed will send the new content directly to you.
Additional information can be found on Wikipedia and w3schools.com.
You'll need an RSS reader before you can use RSS feeds. Outlook 2007 and Internet Explorer 7 both have RSS readers built into them. For a list of readers and programs that contain readers, visit Wikipedia.