RSS stands for Really Simple Syndication and it is a format used to deliver website content; like news, updates or regularly changing web pages, to you the user without needing to visit the website. Updates are sent as soon as the web page that the RSS Feed is related to is updated, and then received by a Feed Reader almost instantly.
Most major browsers, including Firefox, Opera and Safari, can view RSS Feeds in a browser window or tab, and often give you an option to subscribe to that feed from a selection of available Feed Readers. To view a feed on a webpage, simply click the RSS icon when you see it:
If you already have a program installed that is set to handle Feeds, then clicking on a RSS icon may open that program automatically and subscribe to the Feed.
There is a whole variety of Feed Readers available for free. Here is a short list: