Really Simple Syndication(RSS) is a method of distributing links to content in your web site that you'd like others to use. In other words, it's a mechanism to "syndicate" your content.
Say you publish a new web page about a particular topic. You want others interested in that topic to know about it. By listing the page as an "item" in your RSS file, you can have the page appear in front of those who read information using RSS readers. RSS also allows people to easily add links to your content within their own web pages. Bloggers are a huge core audience that especially does this.