Why ask us to type it out when it's explained a million times already elsewhere? http://en.wikipedia.org/wiki/RSS_(file_format)#Usage In short, it's an easy way of keeping up to date with websites. You don't have to visit them if you subscribe to their RSS, the updates come to you.
Basically, it is something that is good for new sites, blogs, and press releases. It is a way to summerize or present information that Yahoo and Google pick up on. Having an RSS feed for your website, or part of your website, can help search engines and people find it. Most blogging software now automatically generates RSS feeds. So do most news sites. You can use some code and disply RSS feeds from another websites, as a source of news, events, or whatever for your visitors.
It is the easy way for our regular visitors to know about the regular updates of our websites, just by making them to do rss subscriptions.