RSS is short for Really Simple Syndication. It's an XML file that can be read by special feed reading software (e.g. OMEA Reader) or a web service you can use from your web browser (IE7 & Firefox). You simply add the address of a web feed to your reader, and it will display the latest headlines, each one with a synopsis and link that will take you to the full story.
This allows you to stay in touch with many different sources of news and information. It's quicker than revisiting a site several times a day to check for updates, and rather than having to look for new headlines, feed readers scan for updates and display them directly.