RSS stands for 'Really Simple Syndication' or 'Rich Site Summary'. It allows web sites to push 'feeds' out to users, or to other web sites. These feeds allow you to see when websites have added new content (e.g. news articles or stories).
To use RSS, you will need a feed reader. Most major browsers (Internet Explorer, Firefox, Safari) allow you to read RSS feeds, but you may want to subscribe to a feed using a stand-alone program that can check automatically when new content has been added.