RSS (Really Simple Syndication) is a mechanism for distributing meta-data-rich news summaries to "subscribers", that is, people who have indicated an interest in receiving those summaries. A given RSS source is usually called a feed, and the user's subscription interface is called a feed reader or feed aggregator. RSS Bandit and the eponymous Feedreader are two open source RSS readers, for example.
There is not space here for a detailed technical explanation of RSS, but you should be aware of two main things. First, the feed reading software is chosen by the subscriber and is the same for all the feeds that subscriber monitors — in fact, this is the major selling point of RSS: that the subscriber chooses one interface to use for all their feeds, so each feed can concentrate just on delivering content. Second, RSS is now ubiquitous, so much so that most people who use it don't even know they're using it. To the world at large, RSS looks like a little button on a web page, with a label saying "Subscribe to this site" or "News feed". You click on the button, and from then on, your feed reader (which may well be an applet embedded in your home page) automatically updates whenever there's news from the site.
This means that your open source project should probably offer an RSS feed (note that many of the canned hosting sites — see “Canned Hosting” — offer it right out of the box). Be careful not to post so many news items each day that subscribers can't separate the wheat from the chaff. If there are too many news events, people will just ignore the feed, or even unsubscribe in exasperation. Ideally, a project would offer separate feeds, one for big announcements, another following (say) events in the issue tracker, another for each mailing list, etc. In practice, this is hard to do well: it can result in interface confusion both for visitors to the project's web site and for the administrators. But at a minimum, the project should offer one RSS feed on the front page, for sending out major announcements such as releases and security alerts.
 See http://www.xml.com/pub/a/2002/12/18/dive-into-xml.html for that.