RSS, acronym for Really Symple Syndacation, is very easily identifiable on web pages by this icon:
RSS "feeds" are a very useful tool for web contents diffusion. You can find a complete explanation on Wikipedia.
Basically, RSS is a very small piece of information (e.g. the first two or three lines of a page) that points to the complete page. When the webmaster updates important pages or publishes new contents on a web site, at the same time, he/she informs us of this, by modifying the RSS page (usually this is automatic, and the page is not viewable while surfing the net).
In PCs, RSS is run by specific software called "RSS reader" (or "aggregator"). When we subscribe to a RSS feed, this software checks the related page at regular intervals (e.g. every half an hour), independently of our activity in that particular time. If that page has changed since the last visit of the RSS reader, and we have not visited it in the mean time, the software will show us (in a way depending on the software itself) the small content of the new RSS feed. If what we read in the RSS reader captures our attention and interest, we can then go on and read the full content on the web.
The usefulness of a RSS feed is quite clear: constant updating on interesting web sites, no waste of time checking them one by one manually, very small use of Internet broadband.
RSS feeds have a worldwide diffusion, so it is possible to choose from many RSS readers. Most web browsers now offer the possibility to use RSS feeds, but this feature can be further improved by installing some free extensions (add-ons). Using the web browser, monitoring of RSS subscribed pages will happen only when the web browser is open, instead, other kinds of software (very cool, nowadays) check constantly the subscribed RSS feeds by just turning on the PC.
An Internet connection is of course required!
Anyway, a google or/and forum search (or any suggestion from a good friend) will give the right solution for everyone and for every operating system.
At the moment I am using Firefox and RSS Ticker extension.