An RSS (Really Simple Syndication) Feed is the information that gets syndicated in your RSS Reader. You can get a website's RSS Feed by clicking on the feed button, which is usually orange and displayed in the right hand side of your browser's address bar or on a site's homepage.
RSS is the format that web sites and blogs can use to send updated headlines to services like Google Reader or Bloglines. It gathers the content you select from a variety of sources and displays it for you in one convenient location.
RSS is a relatively new format, so not all sites syndicate their content.
Still confused? Check out RSS Explained the Oprah Way for more help.