...a form of syndication in which a section of a website is made available for other sites to use. [Most] commonly these days web syndication refers to making Web feeds available from a site so other people can display an updating list of content from it.
You can also read the What is RSS and How Do You Use It? article on About.com to get some more in-depth information and a brief history.
What this means in practice is that I have a “feed” available that you can use to keep up-to-date about postings on this website. You can use the feed in News Reader programs such as NetNewsWire, NewsFire, or FeedDemon. In addition, you could use online services such as Bloglines, My Yahoo!, or NewsGator to keep up with the feed from online. Browsers such as Firefox and Safari RSS will also allow you to subscribe to and view feeds from within the browser.