RSS stands for 'Really Simple Syndication'. RSS feeds are also called 'News feeds' and they allow you to see when websites have added new content. You can get the latest headlines and articles in one place, as soon as they are published, without having to visit the websites that supply the feeds.
You can subscribe to feeds and read them using your browser (e.g. Internet Explorer 7 or Firefox) or by using news reader software.
News Reader software for Windows:
News Reader software for Apple Mac OSX: